// Try Codeforces
// author: Leonardone @ NEETSDKASU
import std.stdio, std.array, std.conv, std.string, std.algorithm;

auto gets() { return readln().chomp(); }
T[] getVals(T)() { return to!(T[])(readln().chomp().split(" ")); }

void main() {
    auto xs = getVals!int();
    auto n = xs[0], k = xs[1];
    auto baloon = gets();
    int[char] mp;
    foreach (b; baloon) {
        mp[b]++;
    }
    int x = 0;
    foreach (int c; mp) {
        x = max(x, c);
    }
    writeln(x <= k ? "YES" : "NO");
}

