#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, a[26], i, j, f = 0;
  memset(a, 0, sizeof(a));
  string s;
  cin >> n >> k;
  cin >> s;
  for (i = 0; i < s.length(); i++) {
    a[s[i] - 'a']++;
  }
  for (i = 0; i < 26; i++) {
    if (a[i] > k) {
      f = 1;
      break;
    }
  }
  if (f == 0) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
  return 0;
}
