#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  int t, i, n, k, flag, c;
  cin >> n >> k;
  cin >> s;
  flag = 0;
  map<char, int> mp;
  map<char, int>::iterator it;
  for (i = 0; i < n; i++) {
    mp[s[i]]++;
  }
  for (it = mp.begin(); it != mp.end(); it++) {
    c = (int)ceil((it->second) * 1.0 / k);
    if (c > 1) {
      flag = 1;
      break;
    }
  }
  if (flag == 0)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
