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.*;
import java.lang.Math.*;

public final class FestiveEvening{

	public static void main(String args[]) throws Exception{

		int temp[] = Br.getIntA(2);
		int N = temp[0], K = temp[1];

		char c[] = Br.getString().toCharArray();

		boolean ans = solve(N, K, c);
		if(ans)
			println("YES");
		else
			println("NO");

	}

	static boolean solve(int N, int K, char c[]){
		boolean arrived[] = new boolean[26];
		int closed[] = new int[26];
		for(int i=0; i<c.length; i++){
			closed[c[i]-65] = (i+1);
		}

		int open = 0;
		for(int i=0; i<c.length; i++){
			if(!arrived[c[i]-65]){
				arrived[c[i]-65] = true;
				open++;
			}

			if(open > K)
				return true;

			if(closed[c[i]-65] == (i+1))
				open--;

		}

		return false;
	}

	// class for Reader
	static class Br {

		final static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		static int getInt() throws IOException{
			return Integer.parseInt(br.readLine());
		}

		static long getLong() throws IOException{
			return Long.parseLong(br.readLine());
		}

		static double getDouble() throws IOException{
			return Double.parseDouble(br.readLine());
		}

		static int[] getIntA() throws IOException{
			String temp[] = br.readLine().split(" ");
			int l = temp.length;

			int a[] = new int[l];

			for(int i=0; i<l; i++){
				a[i] = Integer.parseInt(temp[i]);
			}

			return a;
		}

		static int[] getIntA(int l) throws IOException{
			String temp[] = br.readLine().split(" ");
			int a[] = new int[l];

			for(int i=0; i<l; i++){
				a[i] = Integer.parseInt(temp[i]);
			}

			return a;
		}

		static long[] getLongA(int l) throws IOException{
			String temp[] = br.readLine().split(" ");
			long a[] = new long[l];

			for(int i=0; i<l; i++){
				a[i] = Long.parseLong(temp[i]);
			}

			return a;
		}

		static String getString() throws IOException{
			return br.readLine();
		}

	}

	// class for MaTh libraries
	static class Mt{
		static double precision = 0.00001;

		static double sqrt(double a){
			return newton_raphson(a, a/2);
		}

		static double newton_raphson(double a, double guess){
			if(guess*guess - a <= precision)
				return guess;

			guess = guess - (guess * guess - a)/ (2 * guess);
			return newton_raphson(a, guess);
		}

		static long modPow(long a, long x, long p){

			long ans = 1;
			while(x > 0){
				if(x %2 !=0){
					ans = (ans*a)%p;
				}
				a = (a*a)%p;
				x/=2;
			}

			return (ans%p);
		}
	}

	static<T> void print(T s){
		System.out.print(s);
	}

	static<T> void println(T s){
		System.out.println(s);
	}
}