#include <bits/stdc++.h>
using namespace std;
int ar[150] = {0};
int main() {
  int n, m, i, c = 0;
  string st;
  cin >> n >> m >> st;
  for (i = 0; i < n; i++) {
    ar[(int)st[i]]++;
  }
  for (i = 97; i <= 122; i++) {
    if (ar[i] > m) {
      cout << "NO";
      c++;
      break;
    }
  }
  if (c == 0) {
    cout << "YES";
  }
}
