#include <bits/stdc++.h>
using namespace std;
long long arr[30];
bool guard[30];
int n, k, i, j;
string s;
int main() {
  cin >> n >> k;
  cin >> s;
  for (i = 0; i < s.size(); i++) arr[s[i] - 'A']++;
  bool bad = false;
  for (int i = 0; i < s.size() - 1; i++) {
    if (!guard[s[i] - 'A']) {
      guard[s[i] - 'A'] = true;
      k--;
    }
    if (k < 0) {
      bad = true;
      break;
    }
    arr[s[i] - 'A']--;
    if (arr[s[i] - 'A'] <= 0) k++;
  }
  if (bad)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
