#include <fstream>
#include <cstring>
#include <algorithm>
#include <map>
#define Base 56
#define Mod1 101149
#define Mod2 515423
#define Nmax 16500
using namespace std;
int n,K;
char s[Nmax];
int Pow1[Nmax],a[Mod1];
map < int,int >M;
inline bool Verif(const int k)
{
    int i,Hash1 = 0,Hash2 = 0,P1 = 1,P2 = 1,Hash;
    M.clear();
    for(i = 1;i <= k; ++i)
    {
        Hash1 = (Hash1*Base+s[i])%Mod1;
        Hash2 = (Hash2*Base+s[i])%Mod2;
        if(i < k)
            P1 = (P1*Base)%Mod1,
            P2 = (P2*Base)%Mod2;
    }
    Hash = Hash1 + Hash2;
    ++M[Hash];
    for(i=k+1;i<=n;++i)
    {
        Hash1 = ((Hash1 - (s[i - k] * P1) % Mod1 + Mod1) * Base + s[i]) % Mod1;
        Hash2 = ((Hash2 - (s[i - k] * P2) % Mod2 + Mod2) * Base + s[i]) % Mod2;
        Hash = Hash1 + Hash2;
        if(++M[Hash] >= K)
           return true;
    }
    return false;
}

int main()
{
    ifstream f("substr.in");
    f>>n>>K>>(s+1);
    f.close();
    int Left = 0,Right = n,Middle, sol = 0;
    if(K!=1)
        while(Left<=Right)
        {
            Middle = (Left+Right)>>1;
            if(Verif(Middle))
            {
                sol = Middle;
                Left = Middle + 1;
            }
            else
                Right = Middle-1;
        }
    else
        sol = n;
    ofstream g("substr.out");
    g<<sol<<"\n";
    g.close();
    return 0;
}