#include <bits/stdc++.h>
using namespace std;
int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
int n, k;
int color[200];
char s[120];
int main() {
  n = read();
  k = read();
  scanf("%s", s);
  for (int i = 0; i < n; i++) {
    color[s[i]]++;
  }
  for (int i = 'a'; i <= 'z'; i++) {
    if (color[i] > k) {
      printf("NO");
      return 0;
    }
  }
  printf("YES");
  return 0;
}
