// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;
import std.datetime, std.bigint;

int n, k;
string s;

void main() {
    scan(n, k);
    s = readln.chomp;

    auto t = new int[](n + 10);

    foreach (char ch ; 'A' .. 'Z' + 1) {
        foreach (i ; 0 .. n) {
            if (s[i] == ch) {
                t[i]++;
                break;
            }
        }

        foreach_reverse (i ; 0 .. n) {
            if (s[i] == ch) {
                t[i + 1]--;
                break;
            }
        }
    }

    foreach (i ; 0 .. n + 1) {
        t[i + 1] += t[i];
    }

    debug {
        writeln(t);
    }

    if (t.reduce!max > k) {
        writeln("YES");
    }
    else {
        writeln("NO");
    }
}




void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}