#include <iostream>
#include <fstream>
#include <unordered_map>
#define P 666013
using namespace std;

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

string s;
int n, k;
unordered_map <int, short> M;

int Cod(char ch)
{
    if ('A' <= ch && ch <= 'Z') return ch - 'A';
    if ('a' <= ch && ch <= 'z') return ch - 'a' + 26;
    return ch - '0' + 52;
}

bool Check(int L)
{
    int i, x, p;
    M.clear();
    x = 0;
    p = 1;
    for (i = 1; i < L; i++)
        p = (p * 62) % P;
    for (i = 0; i < L; i++)
        x = (x * 62 + Cod(s[i])) % P;
    M[x] = 1;
    for (; i < n; i++)
    {
        x = (x - Cod(s[i - L]) * p) % P;
        if (x < 0) x += P;
        x = (x * 62 + Cod(s[i])) % P;
        M[x]++;
        if (M[x] == k) return 1;
    }
    return 0;
}

void Search()
{
    int st, dr, mij, l;
    st = 1; dr = n; l = 1;
    while (st <= dr)
    {
        mij = (st + dr) / 2;
        if (Check(mij))
        {
            l = mij;
            st = mij + 1;
        }
        else 
            dr = mij - 1;
    }
    fout << l << "\n";
}

int main()
{
    fin >> n >> k;
    fin >> s;
    Search();
    fout.close();
    return 0;
}