#include <bits/stdc++.h>
using namespace std;
const int N = 300;
string s;
int c[N];
int main() {
  int n, m;
  cin >> n >> m >> s;
  for (char i : s) c[int(i)]++;
  bool ans = true;
  m++;
  for (int i = 0; i < N; i++) ans &= c[i] < m;
  cout << (ans ? "YES" : "NO");
}
