#include <cstdio>
#include <algorithm>
#include <fstream>
#include <iostream>
#include <vector>
#include <queue>
#include <map>
#include <cstring>
#include <string>
#include <set>
#include <stack>
#include <unordered_map>
#define MOD1 100007
#define MOD2 100021
#define BASE 123
#define Nmax 17000
using namespace std;

char buf[Nmax];
int N,K,sol = -1, sol_max =-1;
int run(int sz)
{
	unordered_map<int, int> hash1;
	unordered_map<int, int> hash2;
	string x = "";
	int key1,key2,power1, power2;
	key1 = key2 = 0;
	power1 = power2 = 1;
    for(int i=0;i<sz;++i)
	{
		if(i!=0)
		{
			power1 = (power1*BASE)%MOD1;
			power2 = (power2*BASE)%MOD2;
		}
		
		key1 = (key1*BASE + buf[i])%MOD1;
		key2 = (key2*BASE + buf[i])%MOD2;
	}
    hash1[key1] = 1;
	hash2[key2] = 1;
    int maxim=-1;
     
    for(int i=sz;i<N;++i)
    {
		key1 = (((key1 - power1 * buf[i-sz])%MOD1 + MOD1)*BASE + buf[i])%MOD1;
		key2 = (((key2 - power2 * buf[i-sz])%MOD2 + MOD2)*BASE + buf[i])%MOD2;

        if(hash1.find(key1) != hash1.end() && hash2.find(key2) != hash2.end()) {
			hash1[key1] = hash1[key1] + 1;
			hash2[key2] = hash2[key2] + 1;
		}
        else
            hash1[key1] = hash2[key2] = 1;
        if(hash1[key1] > maxim)
            maxim = hash1[key1];
    }
    return maxim;
}
int main()
{
	ifstream f("substr.in");
	f>>N>>K;
	f.getline(buf,Nmax);
	f.getline(buf,Nmax);
	
	f.close();
	
	int left,right,mid;
	left = 0;right = N-1;
	
	while(left<=right)
	{
		mid = (left+right)/2;
		int x = run(mid);
		
		if(x >= K)
		{
			if(mid > sol)
				sol = mid;
			left = mid+1;
		}
		else
		{
			right = mid-1;
		}
	}
	ofstream g("substr.out");
	g<<sol<<"\n";
	g.close();
	return 0;
}