#include <bits/stdc++.h>
using namespace std;
map<char, int> m;
char c;
int main() {
  int n, k, i;
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cin >> n >> k;
  for (i = 1; i <= n; ++i) cin >> c, ++m[c];
  for (i = 97; i <= 122; ++i)
    if (m[char(i)] > k) {
      cout << "NO";
      return 0;
    }
  cout << "YES";
  return 0;
  return 0;
}
