#include <bits/stdc++.h>
using namespace std;
int main() {
    int n, k; cin >> n >> k;
    string s; cin >> s;
    int cnt[30] = {0};
    for (int i = 0; i < s.size(); i++) {
        cnt[s[i] - 'a']++;
    }
    bool ok = true;
    for (int i = 0; i < 26; i++) {
        if (cnt[i] > k) {
            ok = false; break;
        }
    }
    if (ok) cout << "YES" << endl;
    else cout << "NO" << endl;
    return 0;
}