#include <bits/stdc++.h>
using namespace std;
const long double eps = 1e-16;
const int MOD = (int)1e9 + 9;
const long long INF = (int)1e9 + 10;
const int NHASH = 2;
template <typename T>
vector<T> readVector0(int n) {
  vector<T> res(n);
  for (int i = 0; i < n; i++) cin >> res[i];
  return res;
}
template <typename T>
vector<T> readVector1(int n) {
  vector<T> res(n + 1);
  for (int i = 1; i <= n; i++) cin >> res[i];
  return res;
}
inline long long mod(long long first, long long m = MOD) {
  if (first < 0) first += m;
  return (first % m);
}
template <typename T>
bool sortBy2(const pair<T, T> &a, const pair<T, T> &b) {
  return (a.second < b.second);
}
int n;
vector<long long> p_pow[NHASH];
int p[NHASH];
vector<long long> hp[NHASH];
vector<long long> hs[NHASH];
vector<int> pi;
string s;
long long binPow(long long a, long long b) {
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = mod(res * a);
    a = mod(a * a);
    b >>= 1;
  }
  return res;
}
void getPrefHash(const string &s) {
  for (int k = 0; k < NHASH; k++) {
    p_pow[k] = vector<long long>(n);
    p_pow[k][0] = 1;
    for (int i = 1; i < n; i++) p_pow[k][i] = (p_pow[k][i - 1] * p[k]) % MOD;
    hp[k] = vector<long long>(n + 1, 0);
    for (int i = 0; i < n; i++)
      hp[k][i + 1] = (hp[k][i] + (s[i] - 'a' + 1) * p_pow[k][i]) % MOD;
  }
}
bool checkHashes(const string &s, int i, int j) {
  for (int k = 0; k < NHASH; k++) {
    if (mod(hp[k][i] * binPow(p[k], (j - 1))) != hs[k][j]) return false;
  }
  return true;
}
void prefixFunction(const string &s) {
  for (int i = 1; i < n; i++) {
    int j = pi[i - 1];
    while (j > 0 && s[i] != s[j]) j = pi[j - 1];
    if (s[i] == s[j]) j++;
    pi[i] = j;
  }
}
int main() {
  auto seed = chrono::duration_cast<chrono::nanoseconds>(
                  chrono::steady_clock::now().time_since_epoch())
                  .count();
  mt19937 mt(seed);
  for (int k = 0; k < NHASH; k++) {
    p[k] = uniform_int_distribution<int>(31, 100000)(mt);
  }
  cin >> s;
  n = s.size();
  pi = vector<int>(n, 0);
  getPrefHash(s);
  for (int k = 0; k < NHASH; k++) {
    hs[k] = vector<long long>(n + 1, 0);
    for (int i = 1; i <= n; ++i)
      hs[k][i] = (hp[k][n] - hp[k][i - 1] + MOD) % MOD;
  }
  prefixFunction(s);
  int m_l = 0;
  for (int i = 0; i < n - 1; ++i) {
    if (pi[i] > m_l && checkHashes(s, pi[i], n + 1 - pi[i])) {
      m_l = pi[i];
    }
  }
  if (m_l == 0) {
    cout << "Just a legend";
    return 0;
  }
  cout << s.substr(0, m_l);
  return 0;
}
