dtksub

//#include<bits/stdc++.h>
#include<iostream>
#include<fstream>
#include<vector>
#include<string>
#include<map>

#define long long long
#define nln '\n'

const long I = 1e9, BASE = 311, MOD = 1e9+3;

using namespace std;

// GLobal variables: f1, f2, str

fstream f1,f2;

inline void openf()
{
    f1.open("dtksub.inp", ios:: in);
    f2.open("dtksub.out", ios:: out);
}

inline void closef()
{
    f1.close();
    f2.close();
}

long n, k;
string str;

void data()
{
    f1.tie(0)->sync_with_stdio(0);
    f2.tie(0)->sync_with_stdio(0);
    cin >> n >> k;
    cin >> str;
}

vector<long> poh, has;

void inithash()
{
    // Xay dung BASE mu i
    poh.resize(n, 0);
    poh[0] = 1;
    for (long i = 1; i != n; ++i)
        poh[i] = poh[i-1]*BASE % MOD;

    // Xay dung hash 1 -> i;
    has.resize(n, 0);
    has[0] = str[0] - 'a' + 1;
    for (long i = 1; i != n; ++i)
        has[i] = (has[i-1]*BASE + str[i] - 'a' + 1) % MOD;
}

long hashing(long i, long j)
{
    if (i == 0)
        return has[j];
    return ((has[j] - has[i-1]*poh[j-i+1]%MOD + MOD) % MOD);
}



bool enough(long len)
{
    map<long, long> sav;
    for (long i = 0; i+len-1 != n; ++i)
    {
        long x = hashing(i, i+len-1);
        ++sav[x];
        //cin.get();
        if (sav[x] == k) return 1;
    }
    return 0;
}

long ans = -1;

void process()
{
    int lef = 1, rig = n;
    while (lef <= rig)
    {
        int len = (lef + rig) / 2;
        if (enough(len))
        {
            ans = len;
            lef = len + 1;
        }
        else
            rig = len - 1;
    }
}

void view()
{
    cout << ans << nln;
}

int main()
{
    openf();
    data();
    inithash();
    process();
    view();
    closef();
    return 0;
}