#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string in;
  cin >> in;
  vector<int> sum(200);
  for (char c : in) ++sum[c];
  auto cnt = sum;
  for (char c : in) {
    if (sum[c] == cnt[c]) --k;
    if (k < 0) {
      cout << "YES" << endl;
      return 0;
    }
    --sum[c];
    if (!sum[c]) ++k;
  }
  cout << "NO" << endl;
  return 0;
}
