#include <bits/stdc++.h>
using namespace std;
int A[27], n, raz, k;
int P(char a) { return a - 'a'; }
string x;
int main() {
  cin >> n >> k;
  cin >> x;
  int i = 0;
  while (x[i] != 0) {
    A[P(x[i])]++;
    i++;
  }
  for (int i = 0; i < 26; i++) {
    if (A[i] > k) {
      printf("NO\n");
      return 0;
    }
  }
  printf("YES");
  return 0;
}
