#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  vector<int> a(n + 1);
  for (int i = 0; i < 26; i++) {
    int mi = -1;
    int ma = -1;
    bool flag = 0;
    char target = 'A' + i;
    for (int j = 0; j < n; j++) {
      if (s[j] == target && flag == 0) {
        flag = 1;
        mi = j;
        ma = j;
      } else if (s[j] == target) {
        ma = j;
      }
    }
    if (flag) {
      a[mi] += 1;
      a[ma + 1] -= 1;
    }
  }
  for (int i = 1; i < n; i++) {
    a[i] += a[i - 1];
  }
  for (int i = 0; i < n; i++) {
    if (a[i] > k) {
      cout << "YES" << endl;
      return 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
