#include <fstream>
#include <vector>
#include <map>
#include <climits>
using namespace std;

const int NMAX = 16384;
const int MOD = 1e9 + 7;
const int P = 67;
int h[NMAX + 1];
int inv[NMAX + 1];

int pwr(int a, int b)
{
    int ans = 1;
    while (b)
    {
        if (b & 1)
            ans = 1LL * ans * a % MOD;
        a = 1LL * a * a % MOD;
        b >>= 1;
    }
    return ans;
}

int has(int i, int j)
{
    return 1LL * (h[j] - h[i - 1] + MOD) % MOD * inv[i] % MOD;
}

int n, k;
map <int, int> mp;

bool ok(int val)
{
    mp.clear();
    int i;
    for (i = 1; i + val - 1 <= n; i++)
    {
        int cine = has(i, i + val - 1);
        mp[cine]++;
        if (mp[cine] >= k)
            return 1;
    }
    return 0;
}

int cod(char x)
{
    if (x >= 'a' and x <= 'z')
        return x - 'a' + 1;
    if (x >= 'A' and x <= 'Z')
        return 'z' - 'a' + 1 + x - 'A' + 1;
    return 'z' - 'a' + 1 + 'Z' - 'A' + 1 + x - '0' + 1;
}
signed main()
{
    ifstream cin("substr.in");
    ofstream cout("substr.out");
    int i;
    cin >> n >> k;
    int p = P;
    int needed = pwr(P, MOD - 2);
    inv[0] = 1;
    for (i = 1; i <= n; i++)
    {
        char x;
        cin >> x;
        h[i] = 1LL * (h[i - 1] + 1LL * cod(x) * p % MOD) % MOD;
        inv[i] = 1LL * inv[i - 1] * needed % MOD;
        p = 1LL * p * P % MOD;
    }
    int med, st = 1, dr = n, sol = -1;
    while (st <= dr)
    {
        med = ((st + dr) >> 1);
        if (ok(med))
        {
            sol = med;
            st = med + 1;
        }
        else
            dr = med - 1;
    }
    cout << sol;
}