#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  string s;
  cin >> n;
  cin >> k;
  cin >> s;
  int d;
  int check = 1;
  for (int i = 0; i < n; i++) {
    d = 0;
    for (int j = 0; j < n; j++)
      if (s[i] == s[j]) d++;
    if (d > k) {
      check = 0;
      break;
    }
  }
  if (check == 0)
    cout << "NO";
  else
    cout << "YES";
}
