#include <cstdio>
#include <cstdlib>
#include <map>
#include <algorithm>
using namespace std;
 
#define PI pair<int, int>
#define mp make_pair
#define Mod1 12839
#define Mod2 10007
#define Nmax 16400
 
int C1[Nmax], C2[Nmax], P1[Nmax], P2[Nmax];
int N, K;
char S[Nmax];
map<int, int> Hash;
 
bool Check(int L)
{
    Hash.clear();
    for(int i = 1; i + L - 1 <= N; ++ i)
    {
        int SubC1 = (C1[i + L - 1] - C1[i - 1] * P1[L]) % Mod1;
        while(SubC1 < 0) SubC1 += Mod1;
 
        //int SubC2 = (C2[i + L - 1] - C2[i - 1] * P2[L]) % Mod2;
       // while(SubC2 < 0) SubC2 += Mod2;
 
        Hash[SubC1] ++;
        if(Hash[SubC1] >= K) return 1;
    }
    return 0;
}
 
int main()
{
    freopen("substr.in", "r", stdin);
    freopen("substr.out", "w", stdout);
    int i;
    scanf("%i %i\n", &N, &K);
    gets(S + 1);
    P1[0] = P2[0] = 1;
    for(i = 1; i <= N; ++ i)
    {
        P1[i] = (P1[i - 1] * 107) % Mod1;
       // P2[i] = (P2[i - 1] * 257) % Mod2;
    }
    for(i = 1; i <= N; ++ i)
    {
        C1[i] = (C1[i - 1] * 107 + S[i]) % Mod1;
       // C2[i] = (C2[i - 1] * 257 + S[i]) % Mod2;
    }
    int Left = 0, Right = N, Mid, Ans = -1;
    while(Left <= Right)
    {
        Mid = (Left + Right) / 2;
        if(Check(Mid)) Ans = Mid, Left = Mid + 1;
        else Right = Mid - 1;
    }
    printf("%i\n", Ans);
    return 0;
}