#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  map<char, int> m;
  for (int i = 0; i < n; i++) {
    if (m[s[i]] == 0) {
      if (k == 0) {
        cout << "YES";
        return 0;
      }
      m[s[i]] = 1;
      k--;
    }
    if (find(s.begin() + i + 1, s.end(), s[i]) == s.end()) k++;
  }
  cout << "NO";
}
