G. Пароль

#include <bits/stdc++.h>
#define ull unsigned long long

using namespace std;

const int N = 1e6 + 3;
const ull base = 29;

ull h[N], b[N];
string s;
int n;

ull get(int i, int j) {
    if (i == 0) {
        return h[j];
    }
    return (h[j] - h[i - 1] * b[j - i + 1]);
}

bool check(int len) {
    auto pref = get(0, len - 1);

    for (int i = 1, j = len; j + 1 < n; ++i, ++j) {
        if (pref == get(i, j)) {
            return true;
        }
    }

    return false;
}

int main() {
    ios_base::sync_with_stdio(NULL);
    cin.tie(0); cout.tie(0);
    cin >> s;
    n = s.size();
    b[0] = 1;
    for (int i = 1; i <= n; ++i) {
        b[i] = b[i - 1] * base;
    }
    h[0] = s[0];

    for (int i = 1; i < n; ++i) {
        h[i] = (h[i - 1] * base + s[i]);
    }

    vector<int> d;
    for (int i = 0; i + 1 < n; ++i) {
        if (get(0, i) == get(n - 1 - i, n - 1)) {
            d.push_back(i + 1);
        }
    }

    int low = 0, high = d.size() - 1, ans = 0;
    while (low <= high) {
        int mid = (low + high) / 2;
        int len = d[mid];
        if (check(len)) {
            ans = len;
            low = mid + 1;
        } else {
            high = mid - 1;
        }
    }

    if (ans > 0) {
        cout << s.substr(0, ans);
    } else {
        cout << "Just a legend";
    }
    return 0;
}