#include<bits/stdc++.h>
using namespace std;

int main()
{
    int n,k,ar[1000],c=0,flag=0,count[26] = {0};
    char string[1000];
    scanf("%d %d",&n,&k);
    getchar();
    gets(string);
    while (string[c] != '\0')
    {

        if (string[c] >= 'a' && string[c] <= 'z')
            count[string[c]-'a']++;
        c++;
    }
    for (c = 0; c < 26; c++)
    {


        if(count[c] > k && n <= 2*k)
        {

            flag=1;
        }
        else if(count[c] > k && (n-count[c])<k && )
        {

            flag=1;
        }
    }
    if(flag==0)printf("YES\n");
    else printf("NO\n");

}