#include <fstream>
#include <unordered_map>
#include <string>
#include <deque>
using namespace std;

#define NMax 16400

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

int n,k;
string sir;
int sol;

inline bool valid(int val)
{
    unordered_map<deque<char>, int> h;
    unordered_map<deque<char>, int>::iterator it;
    int viz[NMax];
    for(int i=0;i<=n;++i) viz[i] = 0;
    int key = 0;
    deque<char> aux;
    for(int i=0;i<val;++i) aux.push_back(sir[i]);
    h[aux] = ++key;

    for(int i=val;i<sir.size();++i)
    {
        aux.pop_front();
        aux.push_back(sir[i]);
        it = h.find(aux);
        if(it != h.end())
        {
            int x = (*it).second;
            if(++viz[x] == k) return true;
        }
        else h[aux] = ++key;
    }

    return false;
}

inline void cautbin()
{
    int st = 1, dr = n;
    while(st<=dr)
    {
        int mij = ((st+dr)>>1);
        if(valid(mij))
        {
            sol = max(sol, mij);
            st = mij + 1;
        }
        else dr = mij - 1;
    }
}

int main()
{
    f>>n>>k>>sir;
    --k;
    cautbin();
    g<<sol<<"\n";

    f.close();
    g.close();
    return 0;
}