#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m;
  int a[256] = {0};
  string char_1;
  cin >> n >> m;
  cin >> char_1;
  for (int i = 0; i <= n; i++) {
    a[char_1[i]]++;
  }
  int f = 1;
  for (int i = 1; i <= 255; i++) {
    if (a[i] > m) f = 0;
  }
  if (f == 0) {
    cout << "NO";
  } else {
    cout << "YES";
  }
  return 0;
}
