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.util.*;
import java.lang.*;
import java.io.*;
import java.math.*;
/* Name of the class has to be "Main" only if the class is public. */
public class Codechef
{
    static BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    
    //StringTokenizer st=new StringTokenizer(br.readLine());
    //ArrayList<Integer> list = new ArrayList<Integer>();
    //int n=Integer.parseInt(st.nextToken());
    //Long n=Long.parseLong(st.nextToken());
    //long n=Long.parseLong(br.readLine());
    //int n=Integer.parseInt(br.readLine());
    //int n=scan.scanInt();
    /*int t=Integer.parseInt(br.readLine());
        for(;t!=0;t--){
            
        }*/
    
	public static void main (String[] args) throws java.lang.Exception
	{
		int i;
	    Scan scan=new Scan();
		Print print=new Print();
		int n=scan.scanInt();
		int k=scan.scanInt();
		
		String s=scan.scanString();
		
		int arr[] = new int[26];
		
		for(i=0;i<26;i++)
		arr[i]=-1;
		
		for(i=0;i<s.length();i++)
		{
		    arr[s.charAt(i)-'A']=i;
		}
		ArrayList<Integer> list = new ArrayList<Integer>();
		int max=0,f=0;
		for(i=0;i<s.length();i++)
		{
		    int a=s.charAt(i)-'A';
		    
		    if(list.contains(a))
		    {}
		    else
		    {
		        list.add(a);
		        max++;
		        if(max>k)
		        {f=1;break;}
		    }
		    
		    if(arr[a]==i)
		    max--;
		    
		    //System.out.println(max);
		}
		if(f==1)
		System.out.println("YES");
		else
		System.out.println("NO");
	}
}
class Scan
{
	private byte[] buf=new byte[1024];
	private int index;
	private InputStream in;
	private int total;
	public Scan()
	{
		in=System.in;
	}
	public int scan()throws IOException
	{
		if(total<0)
		throw new InputMismatchException();
		if(index>=total)
		{
			index=0;
			total=in.read(buf);
			if(total<=0)
			return -1;
		}
		return buf[index++];
	}
	public int scanInt()throws IOException
	{
		int integer=0;
		int n=scan();
		while(isWhiteSpace(n))
		n=scan();
		int neg=1;
		if(n=='-')
		{
			neg=-1;
			n=scan();
		}
		while(!isWhiteSpace(n))
		{
			if(n>='0'&&n<='9')
			{
				integer*=10;
				integer+=n-'0';
				n=scan();
			}
			else throw new InputMismatchException();
		}
		return neg*integer;
	}
	public double scanDouble()throws IOException
	{
		double doub=0;
		int n=scan();
		while(isWhiteSpace(n))
		n=scan();
		int neg=1;
		if(n=='-')
		{
			neg=-1;
			n=scan();
		}
		while(!isWhiteSpace(n)&&n!='.')
		{
			if(n>='0'&&n<='9')
			{
				doub*=10;
				doub+=n-'0';
				n=scan();
			}
			else throw new InputMismatchException();
		}
		if(n=='.')
		{
			n=scan();
			double temp=1;
			while(!isWhiteSpace(n))
			{
				if(n>='0'&&n<='9')
				{
					temp/=10;
					doub+=(n-'0')*temp;
					n=scan();
				}
				else throw new InputMismatchException();
			}
		}
		return doub*neg;
	}
	public String scanString()throws IOException
	{
		StringBuilder sb=new StringBuilder();
		int n=scan();
		while(isWhiteSpace(n))
		n=scan();
		while(!isWhiteSpace(n))
		{
			sb.append((char)n);
			n=scan();
		}
		return sb.toString();
	}
	private boolean isWhiteSpace(int n)
	{
		if(n==' '||n=='\n'||n=='\r'||n=='\t'||n==-1)
		return true;
		return false;
	}
}
 
class Print
{
	private final OutputStream out;
	/*public Print(OutputStream outputStream)
	{
		writer=new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
	}*/
	public Print()
	{
		this.out=System.out;
	}
	public void print(String str)throws IOException
	{
		//buf=str.getBytes();
		for (int i = 0; i < str.length(); i++)
		{
			/*if (i != 0)
			out.write(' ');*/
			out.write(str.charAt(i));
		}
	}
	public void printLine(String str)throws IOException
	{
		print(str);
		out.write('\n');
	}
	public void close()throws IOException
	{
		out.close();
	}
} 
