#include <bits/stdc++.h>
using namespace std;
char a[10000];
int d[30];
int main() {
  int n, k;
  cin >> n >> k;
  scanf("%s", a);
  for (int i = 0; i < n; i++) d[a[i] - 'a']++;
  int r = 0;
  for (int i = 0; i < 26; i++)
    if (d[i] > k) {
      r = 1;
      break;
    }
  if (r == 1)
    cout << "NO";
  else
    cout << "YES";
  return 0;
}
