#include <bits/stdc++.h>
int main() {
  int n, k;
  char x[105];
  while (~scanf("%d%d", &n, &k)) {
    int i;
    int sum[105] = {0};
    getchar();
    for (i = 0; i < n; i++) {
      scanf("%c", &x[i]);
      sum[(x[i] - 97)]++;
    }
    int Max = 0;
    for (i = 0; i < 27; i++) {
      if (sum[i] > Max) Max = sum[i];
    }
    if (Max <= k)
      printf("YES\n");
    else
      printf("NO\n");
  }
}
