#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;
int n, k;
string s;
int b[28];
int main() {
	cin >> n >> k;
	cin >> s;
	for (int i = 0; i < n; i++) {
		b[s[i] - 'a']++;
	}
	for (int i = 0; i < 28; i++) {
		if (b[i] > k && b[i] % k) { puts("no"); return 0; }

	}
	puts("yes"); return 0;

	return 0;
}