#include <bits/stdc++.h>
using namespace std;
int a[30];
int main() {
  int n, k;
  char s;
  cin >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> s;
    a[(int)(s)-96]++;
  }
  int mx = 0;
  for (int i = 0; i < 30; i++) {
    mx = max(mx, a[i]);
  }
  if (mx > k) {
    cout << "NO";
  } else
    cout << "YES";
}
