#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, cnt = 1;
  cin >> n >> k;
  string str;
  cin >> str;
  for (int i = 0; i < str.size(); i++) {
    for (int j = i + 1; j < str.size(); j++) {
      if (str[j] == str[i]) cnt++;
      if (cnt > k) {
        cout << "NO\n";
        return 0;
      }
      if (j == n - 1) cnt = 1;
    }
  }
  cout << "YES" << endl;
  return 0;
}
