#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, i, j, c = 1, flag;
  string s;
  char temp;
  cin >> n >> k >> s;
  for (i = 0; i < n; i++) {
    for (j = i + 1; j < n; j++) {
      if (s[i] == s[j]) {
        c++;
      }
    }
    if (c <= k) {
      flag = 1;
      c = 1;
    } else {
      flag = 0;
      break;
    }
  }
  if (flag == 1)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
