#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;
}