Anionic Monoazo Dyes The present invention relates to novel anionic monoazo dyes, a process for their preparation and the use of these dyes for dyeing natural or synthetic materials, in particular, paper. Monoazo dyes based on coupling reactions of diazotised aromatic amines with 1,3,5-triazinyl-l-acid derivatives have previously been described, for example, in EP 548,795, solely in the form of reactive dyes for cotton. Furthermore, in recent years, the use of concentrated aqueous solutions of dyes has gained importance because of the advantages possessed by such solutions when compared with dyes in powder form. The use of solutions avoids the difficulties associated with dust fomiation and releases the user from the time-consuming and frequently difficult dissolving of the dye powder in water. The use of concentrated solutions was also prompted by the development of continuous dyeing processes for paper, since it is convenient in these processes to meter the solution directly into the pulp stream or to add it at some other suitable point of the papermaking process. Surprisingly, it has now been found that anionic dyes based on this chromophoric system are especially valuable for use in dyeing paper, since they possess highly desirable yellowish-red shades. Such shades of dyeings have, hitherto, only been attainable with difficulty, since no single dyestuff has been available and it has been necessary to incorporate mixtures of yellow and red dyes to obtain such shades. Furthermore, the dyes of the present invention exhibit high degrees of exhaustion under particular dyeing conditions, resulting in dyeings of exceptional brilliance not obtainable by the use of mixtures. In addition the dyes of the invention exhibit excellent water-solubility, thus enabling the ready preparation of concentrated liquid selling grades. Accordingly, the invention relates to compounds of the formula in which A represents a 1- or 2-naphthyl residue, which is substituted by a total of one or two sulphonic and/or carboxylic acid groups, preferably a 1- or 2-naphthyl mono- or disulphonic acid or a 1 - or 2-naphthyl monocarboxylic acid residue, R1 represents hydrogen or C1-C4alkyl, each D, and D2, independently of the other, represent either an amino acid residue resulting from removal of a hydrogen atom from the amino group of the amino acid or the residue -NR2R3, in which each R2 and R3, independently of the other, represent hydrogen, C1-C4alkyl, Ca-CealkyI which is substituted by hydroxy, halogen or cyano, phenyl which is unsubstituted or monosubstituted by hydroxy, halogen, SO3H, Ci-C4alkyl or Ci-C4alkoxy or, alternatively, R2 and R3, together with the nitrogen atom to which they are connected, complete a saturated, 5- or 6-membered ring which may contain, in addition to the nitrogen atom, one nitrogen or oxygen atom and which may be further substituted and n isOor 1. More prefen-ed compounds of formula (1) are those in which R1 represents hydrogen D1and D2, independently of the other, is an amino acid residue residue resulting from removal of a hydrogen atom from the amino group of the amino acid and which is derived from glycine, alanine, serine, cysteine, phenylalanine, tyrosine (4-hydroxyphenylalanine), diiodotyrosine, tryptophan (P-indolylalanine), histidine ((P-imidazolylalanine), a-aminobutyric acid, methionine, valine (a-amlnoisovaleric acid), norvaline, leucine (a- aminoisocaproic acid), isoleucine (a-amino-P-methylvaleric acid), norleucine (a-amino-n-caproic acid), arginine, ornithine (a,5-diaminovaleric acid), lysine (a,e-diaminocaproic acid), aspartic acid (aminosuccinic acid), glutamic acid (a-aminoglutaric acid), threonine and hydroxyglutamic acid as well as mixtures and optical isomers thereof or from iminodiacetic acid, a residue -NR2R3, in which each R2 and R3, independently of the other, represent hydrogen, C2-C4hydroxyalkyl, phenyl, which is unsubstituted or monosubstituted by SO3H or, alternatively, a morpholino, piperidino or pyrrolidine residue. Especially preferred compounds of formula (1) are those in which A represents a 1-naphthyl-2-, 3-, 4-, 5-, 6-, 7- or 8-sulphonic acid, a 2-naphthyl-1-, 5-, 6- or 7-suiphonic acid, a 2-naphthyl-1-, 3- or 6-carboxylic acid, a 1-naphthyl-3,8- or 4,8-disulphonic acid or a 2-naphthyl-1,5-, 3,6-, 4,8- or 6,8-disulphonic acid residue and each D1 and D2, independently of the other, is an amino acid residue from which a hydrogen atom on the amino group has been removed and which is derived from glycine, alanine, serine, phenylalanine, aspartic acid (aminosuccinic acid) or glutamic acid (a-aminoglutaric acid), a residue -NR2R3, in which each R2 and R3, independently of the other, represent hydrogen, C2-C3hydroxyalkyI, phenyl, which is unsubstituted or monosubstituted by SO3H or, alternatively, a morpholino residue. Most especially preferred compounds of formula (1) are those in which A represents a 1-naphthyl-2-, 3-, 4-, 5-, 6-, 7- or 8-sulphonic acid, a 2-naphthyl-1-. 5-, 6- or 7- sulphonic acid, a 2-naphthyl-1-, 3- or 6-carboxylic acid, a 1-naphthyl-3,8- or 4,8- disulphonic acid or a 2-naphthyl-1,5-, 3,6-, 4,8- or 6,8-disulphonic acid residue, most especially, when n is 0, a 2-naphthyl-6- or 7-sulphonic acid residue and, when n is 1, a 1-naphthyl-4-sulphonic acid, 2-naphthyl-6-sulphonic acid or a 2-naphthyl-1,5- disuiphonic acid residue, Ri represents hydrogen and both D1 and D2 represent the group -NHCH2CH2OH. The sulphonic and/or carboxylic acid groups present in compounds of formula (1) may be present either in the form of the free acid or in the salt form, SO3M and/or CO2M. M is preferably one equivalent of a colourless cation, typically lithium, sodium, potassium, ammonium or the protonated form of a C4-Ci2trialkylamine, C1C12diamine, C2-C12-alkanolamine or of a polyglycol amine, conveniently, triethanolamine trisglycol ether, or mixtures of such cationic species. M as a protonated C4-C,2trialkylamine may, for example, be a protonated N-ethyl-dimethylamine, N,N-diethylmethylamine, tri-n-propylamine, tri-n-butylamine, tri-isobutylamine, and, preferably, triethylamine or triisopropylamine. M as a protonated C4-Ci2diamine may, for example, be ethylenediamine, or 1,3-diaminopropane, in which one or both nitrogen atoms are additionally substituted by one or two C,-C4aikyl radicals, preferably methyl or ethyl radicals. M is preferably an N,N-dialkylethylenediamine orN,N-diaIkyl-1,3-diaminopropane. Illustrative examples are: N-ethylethylenediamine, N,N-dimethylethylenediamine, N.N'-dimethylethylenediamine, N,N-diethylethylenediamine, 3-dimethylamino-1-propylamine or 3-dlethylamino-1-propylamine. M as a protonated C2-Ci2alkanolamine may be the protonated form of a monoalkanolamine, dialkanolamine, monoalkanolmonoalkylamine, monoalkanoldialkylamine, dialkanolalkylamine or trialkanolamine or a mixture of different protonated alkanolamines. Illustrative examples are: protonated 2-aminoethanol, bis(2-hydroxyethyl)amine, N-(2-hydroxyethyl)dimethylamine, N-(2-hydroxyethyl)diethylamine, N,N-bis(2-hydroxyethyl)ethylamine or tris(2-hydroxyethyl)-amine. Within the scope of the definitions of R, as Ci-C4alkyl and R2 and/or R3 as Ci-C4alkyl and/ or C2-C6alkyl which is substituted by hydroxy, halogen or cyano, these alkyl radicals may be branched or unbranched, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, t-butyl, 2-ethylbutyl, n-pentyl, isopentyl, 1-methylpentyl, 1,3-dimethylbutyl or n-hexyl. Similarly, C,-C4alkoxy may be, for example, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, sec-butoxy, isobutoxy or t-butoxy. Halogen in the above formulae and radicals is iodine, bromine, fluorine or, especially, chlorine. The dyes of formula (1) of the invention may be prepared by known methods, for example by reacting the diazonium salt of an amine of the formula A-NHz (2) with either 2-amino- or 2-Ci-C4alkylamino-5-hydroxynaphthalene-7-sulphonic acid (where n=0) or with 2-(4-amino- or 4-Ci-C4alkylaminobenzoyl)amino- or C1i-C4alkylamino-5-hydroxynaphthalene-7-sulphonic acid (where n=1), reaction with cyanuric chloride and subsequent sequential reaction of the dichloro intermediate with amines D,H and D2H or, alternatively, reacting 2-amino- or 2-C,-C4alkylamino-5-hydroxynaphthalene-7-sulphonic acid (where n=0) or 2-(4-amino- or 4-Ci-C4all