#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, flag = 1;
  cin >> n >> k;
  string s;
  cin >> s;
  int a[26] = {};
  for (int i = 0; i < n; i++) {
    int x = s[i] - 97;
    a[x]++;
  }
  for (int i = 0; i < 26; i++) {
    if (a[i] > k) {
      flag = 0;
      break;
    }
  }
  if (flag)
    cout << "YES\n";
  else
    cout << "NO\n";
  return 0;
}
