#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, i, *d;
  char *s;
  cin >> n >> k;
  s = new char[n];
  for (i = 0; i < n; i++) cin >> s[i];
  d = new int[n];
  for (int i = 0; i < n; i++) d[i] = 0;
  for (i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
      if (s[i] == s[j]) {
        d[i]++;
        if (d[i] > k) {
          cout << "NO";
          exit(0);
        }
      } else if (s[i] == s[j] && j < i) {
        d[i] = d[j];
        break;
      }
    }
  }
  cout << "YES";
  return 0;
}
