433 3
FZDDHMJGBZCHFUXBBPIEBBEFDWOMXXEPOMDGSMPIUZOMRZQNSJAVNATGIWPDFISKFQXJNVFXPHOZDAEZFDAHDXXQKZMGNSGKQNWGNGJGJZVVITKNFLVCPMZSDMCHBTVAWYVZLIXXIADXNYILEYNIQHKMOGMVOCWGHCWIYMPEPADSJAAKEGTUSEDWAHMNYJDIHBKHVUHLYGNGZDBULRXLSAJHPCMNWCEAAPYMHDTYWPADOTJTXTXUKLCHWKUSZRHEKQEFPVJEJJHRWCKYOIWALRTIBUMNOCRXLSIKQCJVQXEPGOHRUDJDKMUUUDORURWXJNVRVMNOUNRFKSVMTMZGOIJLXEPAMVGESOADYIGZXRBJDIWKNOWTCSROAQTBECHTOZVSQUOOJRZIBAUHMKAXDCIMDZJFMABGRNTGPUJAUNFPFWCJG
 6 2
ABCABC
 41 19
TMEYYIIELFDCMBDKWWKYNRNDUPRONYROXQCLVQALP
 5 2
ABACA
 377 3
EADADBBBBDEAABBAEBABACDBDBBCACAADBEAEACDEAABACADEEDEACACDADABBBBDDEECBDABACACBAECBADAEBDEEBDBCDAEADBCDDACACDCCEEDBCCBBCEDBECBABCDDBBDEADEDAEACDECECBEBACBCCDCDBDAECDECADBCBEDBBDAAEBCAAECCDCCDBDDEBADEEBDCAEABBDEDBBDDEAECCBDDCDEACDAECCBDDABABEAEDCDEDBAECBDEACEBCECEACDCBABCBAAEAADACADBBBBABEADBCADEBCBECCABBDDDEEBCDEBADEBDAAABBEABADEDEAEABCEEBEEDEAEBEABCEDDBACBCCADEBAAAAAEABABBCE
 3 26
AAB
 27 1
ABCDEFGHIJKLMNOPQRSTUVWXYZA
 73 2
DEBECECBBADAADEAABEAEEEAEBEAEBCDDBABBAEBACCBEEBBAEADEECACEDEEDABACDCDBBBD
 7 2
ABCDBCD
 5 1
AZAZA
 44 15
HGJIFCGGCDGIJDHBIBGAEABCIABIGBDEADBBBAGDFDHA
 5 2
ABCAB
 8 3
ABCBCDCA
 26 1
ABCDEFGHIJKLMNOPQRSTUVWXYZ

YES
 YES
 NO
 NO
 YES
 NO
 YES
 YES
 YES
 YES
 NO
 YES
 NO
 NO

import java.io.IOException;
import java.util.*;

public class Program2
{
    public static void main(String[] args)
	{
        FasterScanner sc = new FasterScanner();
		int n = sc.nextInt();
        int k = sc.nextInt();
        String doors = sc.nextLine();
        boolean flag = true;
        int array[] = new int[26];
        boolean guards[] = new boolean[26];
        for(int i = 0; i < n; i++) {
            array[(int)doors.charAt(i) - 65]++;
        }
        //System.out.printf("%s\n", Arrays.toString(array));
        for(int i = 0; i < n; i++) {
            int j = (int)doors.charAt(i) - 65;
            //System.out.printf("%s\n", Arrays.toString(array));
            array[j]--;
            if(guards[j] == true) {
                if(array[j] == 0) {
                    guards[j] = false;
                    k++;
                }
            } else if(k != 0 && array[j] != 0) {
                guards[j] = true;
                k--;
            } else if(k == 0) {
                System.out.println("YES");
                flag = false;
                break;
            }
        }
        if(flag) {
            System.out.println("NO");
        }
    }
    
	public static class FasterScanner 
	{
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public int read() 
		{
			if (numChars == -1)
				throw new InputMismatchException();
			if (curChar >= numChars) 
			{
				curChar = 0;
				try 
				{
					numChars = System.in.read(buf);
				} 
				catch (IOException e) 
				{
					throw new InputMismatchException();
				}
				if (numChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

		public String nextLine() 
		{
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = read();
			} while (!isEndOfLine(c));
			return res.toString();
		}

		public String nextString() 
		{
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		public long nextLong()
		{
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-')
			{
				sgn = -1;
				c = read();
			}
			long res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}

		public int nextInt() 
		{
			int c = read();
			while (isSpaceChar(c))
				c = read();
			int sgn = 1;
			if (c == '-') 
			{
				sgn = -1;
				c = read();
			}
			int res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = read();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
	        
	    public int[] nextIntArray(int n)
		{
	        int[] arr = new int[n];
	        for (int i = 0; i < n; i++) 
			{
	            arr[i] = nextInt();
	        }
	        return arr;
	    }
        
		public long[] nextLongArray(int n) 
		{
		    long[] arr = new long[n];
		    for (int i = 0; i < n; i++) 
			{
		        arr[i] = nextLong();
		    }
		    return arr;
		}
		
		public String[] nextStringArray(int n) 
		{
		    String[] arr = new String[n];
		    for (int i = 0; i < n; i++) 
			{
		        arr[i] = nextString();
		    }
		    return arr;
		}

	    private boolean isSpaceChar(int c) 
		{
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) 
		{
			return c == '\n' || c == '\r' || c == -1;
		}
	}
}