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.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class B {
    
    public static void main(String[] args) throws IOException {
        br = new BufferedReader(new InputStreamReader(System.in));
        st = new StringTokenizer("");
        sb = new StringBuilder();
        
        int n = nxtInt();
        int k = nxtInt();
        char[] guests = nxtCharArr();
        
        int[] charArr = new int[26];
        boolean[] vis = new boolean[26];
        for(int i = 0; i < n; i++) {
        	int num = guests[i] - 'A';
        	charArr[num]++;
        }
        
        for(int i = 0; i < n; i++) {
        	int index = guests[i] - 'A';
        	if (!vis[index]) {
        		vis[index] = true;
        		k--;
        	}
        	if (k < 0) {
        		System.out.println("YES");
        		return;
        	}
        	
        	charArr[index]--;
        	if (charArr[index] == 0) {
        		k++;
        	}
        }
        System.out.println("NO");
    }
    
    static BufferedReader br;
    static StringTokenizer st;
    static StringBuilder sb;
    static FileReader fr;

    static String nxtTok() throws IOException {
        while (!st.hasMoreTokens()) {
            String s = br.readLine();
            if (s == null)
                return null;
            st = new StringTokenizer(s.trim());
        }
        return st.nextToken();
    }

    static int nxtInt() throws IOException {
        return Integer.parseInt(nxtTok());
    }

    static long nxtLng() throws IOException {
        return Long.parseLong(nxtTok());
    }

    static double nxtDbl() throws IOException {
        return Double.parseDouble(nxtTok());
    }

    static int[] nxtIntArr(int n) throws IOException {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nxtInt();
        return a;
    }

    static long[] nxtLngArr(int n) throws IOException {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nxtLng();
        return a;
    }

    static char[] nxtCharArr() throws IOException {
        return nxtTok().toCharArray();
    }       
}