#include <fstream>
#include <vector>
#include <bitset>
#include <tr1/unordered_map>
#include <map>

using namespace std;
using namespace tr1;

const char infile[] = "substr.in";
const char outfile[] = "substr.out";

ifstream cin(infile);
ofstream cout(outfile);

const int MAXN = 17000;
const int oo = 0x3f3f3f3f;
const int MOD = 16385000;

typedef vector<int> Graph[MAXN];
typedef vector<int> :: iterator It;

const inline int min(const int &a, const int &b) { if( a > b ) return b;   return a; }
const inline int max(const int &a, const int &b) { if( a < b ) return b;   return a; }
const inline void Get_min(int &a, const int b)    { if( a > b ) a = b; }
const inline void Get_max(int &a, const int b)    { if( a < b ) a = b; }

int N, K;
char S[MAXN];
unordered_map <int, int> Hash;

inline bool Check(int x) {
    Hash.clear();
    int last = 1, _hash = 0;
    for(int i = 0 ; i < x ; ++ i) {
        _hash = ( (_hash << 5) + _hash  + S[i]) % MOD;
        last = ( (last << 5) + last) % MOD;
    }
    ++ Hash[_hash];
    for(int i = x ; i < N ; ++ i) {
        _hash = ((_hash << 5) + _hash + S[i]) % MOD;
        _hash -= ( last * S[i - x] ) % MOD;
        if( _hash < 0 )
            _hash += MOD;
        if(++ Hash[_hash] >= K)
            return 1;
    }
    return 0;
}

int main() {
    cin >> N >> K >> S;
    int st = 0, dr = N, Ans = N;
    if(K == 1)
        goto _end;
    while(st <= dr) {
        int mid = ((st + dr) >> 1);
        if(Check(mid)) {
            Ans = mid;
            st = mid + 1;
        }
        else dr = mid - 1;
    }
    _end:
    cout << Ans << '\n';
    cin.close();
    cout.close();
    return 0;
}