#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k;
  string colors;
  cin >> n >> k;
  cin >> colors;
  sort(colors.begin(), colors.end());
  int count = 0;
  char currentColor;
  bool canGiveOut = true;
  for (int i = 0; i < n && canGiveOut == true; ++i) {
    if (!count) {
      currentColor = colors[i];
      count = 1;
    } else if (colors[i] == currentColor) {
      ++count;
    } else {
      count = 1;
      currentColor = colors[i];
    }
    if (count > k) {
      canGiveOut = false;
    }
  }
  cout << (canGiveOut ? "YES" : "NO");
  return 0;
}
