#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll INF = 1e6;
int c[50];
int main() {
  string str;
  ll n, k;
  cin >> n >> k;
  cin >> str;
  int mx = 0;
  for (int i = 0; i < n; i++) {
    int alfa = str[i] - 'a';
    c[alfa]++;
    mx = max(mx, c[alfa]);
  }
  if (mx > k) {
    cout << "NO\n";
  } else {
    cout << "YES\n";
  }
}
