#include <bits/stdc++.h>
using namespace std;
char s[1001000];
int st[27], f[1001000] = {0};
int main() {
  int n, k, l;
  cin >> n >> k;
  scanf("%s", s);
  for (int i = 0; i < 26; ++i) st[i] = -1;
  l = strlen(s);
  for (int i = 0; i < l; ++i) {
    if (st[s[i] - 'A'] == -1) {
      f[i]++;
    }
    st[s[i] - 'A'] = i;
  }
  for (int i = 0; i < 26; ++i) {
    if (st[i] != -1) f[st[i] + 1]--;
  }
  int num = 0;
  for (int i = 0; i < l; ++i) {
    num += f[i];
    if (num > k) {
      cout << "YES" << endl;
      return 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
