#include <iostream>
#include <fstream>
#include <unordered_map>

using namespace std;

ifstream fin("substr.in");
ofstream fout("substr.out");

const int MOD = 1e9 + 7;
const int BASE = 29;

int n, k;
string s;

unordered_map <int, int> umap;

bool check(int sz){
    umap.clear();

    int powB = 1;
    for(int i = 1; i <= sz; i++)
        powB = (1ll * powB * BASE) % MOD;

    int hash = 0;
    for(int i = 1; i <= n; i++){
        hash = (1ll * hash * BASE + (s[i] - 'a')) % MOD;
        if(i >= sz){
            hash = (1ll * hash + MOD - ((1ll * powB * (s[i - sz] - 'a')) % MOD)) % MOD;
            if(umap[hash] + 1 >= k)
                return true;
            umap[hash]++;
        }
    }

    return false;
}

int main(){

    fin >> n >> k;
    fin >> s, s = "$" + s;

    s[0] = 'a';

    int st = 1, dr = n, ans = 0;
    while(st <= dr){
        int mij = (st + dr) / 2;
        if(check(mij))
            st = mij + 1, ans = mij;
        else
            dr = mij - 1;
    }

    fout << ans << '\n';

    return 0;
}