#include <bits/stdc++.h>   using namespace std;   char mp[1000];   int main() {     int n, k;     cin >> n >> k;     string s;     cin >> s;     for (int i = 0; i < n; i++) {       mp[s[i] -  a ]++;       if (mp[s[i] -  a ] > k) {         cout <<  NO ;         return 0;       }     }     cout <<  YES ;   }   