#include <bits/stdc++.h>
using namespace std;
int a[255], n, k;
string s;
void setup() {}
void solve() {
  cin >> n >> k >> s;
  for (int i = 0; i <= n - 1; ++i) a[s[i]]++;
  for (int i = 97; i <= 122; ++i)
    if (a[i] > k) {
      cout << "NO";
      return;
    }
  cout << "YES";
}
int main() {
  setup();
  solve();
  return 0;
}
