#include <bits/stdc++.h>
using namespace std;
int a[30];
string b;
int main() {
  int n, k;
  cin >> n >> k;
  cin >> b;
  for (int i = 0; i < b.size(); i++) a[b[i] - 96]++;
  sort(a + 1, a + 27);
  reverse(a + 1, a + 27);
  if (a[1] <= k)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
