#include <bits/stdc++.h>
using namespace std;
map<char, int> cnum;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int N, K;
  cin >> N >> K;
  string colors;
  cin >> colors;
  for (int i = 0; i < N; i++) {
    char cur = colors[i];
    cnum[cur]++;
  }
  for (auto f : cnum) {
    if (f.second > K) {
      printf("NO\n");
      return 0;
    }
  }
  printf("YES\n");
  return 0;
}
