#include <bits/stdc++.h>
using namespace std;
int n, k;
string s;
int arr[30];
int main() {
  cin >> n >> k;
  cin >> s;
  memset(arr, 0, sizeof(arr));
  for (int i = 0; i < n; i++) {
    arr[s[i] - 'a']++;
  }
  int f = 0;
  for (int i = 0; i <= 25; i++) {
    if (arr[i] > k) {
      f = 1;
      break;
    }
  }
  if (f)
    cout << "NO" << endl;
  else
    cout << "YES" << endl;
  return 0;
}
