#include <iostream>
#include <fstream>
#include <tr1/unordered_map>

using namespace std;

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

const int MAXN = 16384 + 1;
const int power = 71;
typedef unsigned long long ULL;

int N, K;
char S[MAXN];
ULL P[MAXN];
tr1 :: unordered_map <ULL, int> M;

void Preprocess ()
{
    int i;

    P[1] = power;
    for (i = 2; i <= N; i ++)
        P[i] = P[i - 1] * power;
}

bool ok (int len)
{
    int i;
    int NrMax = 1;
    ULL hash = 0;

    for (i = 0; i < len; i ++)
        hash = hash * power + S[i];

    M[hash] ++;

    for (i = len; i < N; i ++){
        hash = hash - S[i - len] * P[len - 1];
        hash = hash * power + S[i];

        M[hash] ++;
        if (M[hash] > NrMax)
            NrMax = M[hash];
    }

    return (NrMax >= K);
}

int BSearch ()
{
    int i, step;

    for (step = 1; step < N; step <<= 1 );
    for (i = 0; step; step >>= 1)
        if (i + step <= N && ok (i + step))
            i += step;

    return i;
}

int main()
{
    in >> N >> K;
    in >> S;

    Preprocess ();
    out << BSearch ();

    return 0;
}