#include <bits/stdc++.h>
using namespace std;
vector<int> z_func(const string &s) {
  vector<int> z(s.size());
  int l = 0, r = 0;
  for (int _n(s.size()), k(1); k < _n; k++) {
    if (k <= r) z[k] = min(z[k - l], r - k + 1);
    while (k + z[k] < s.size() && s[k + z[k]] == s[z[k]]) ++z[k];
    if (k + z[k] - 1 > r) l = k, r = k + z[k] - 1;
  }
  return z;
}
int main() {
  string s;
  cin >> s;
  const int N = s.size();
  if (N <= 2) {
    cout << "Just a legend" << endl;
    return 0;
  }
  {
    set<char> ch((s).begin(), (s).end());
    if (ch.size() == 1) {
      cout << s.substr(0, N - 2) << endl;
      ;
      return 0;
    }
  }
  vector<int> z = z_func(s);
  int mx = 0;
  for (int _n(N), i(1); i < _n; i++) mx = max(mx, min(z[i], N - 1 - i));
  int ans = -1;
  for (int _n(N), i(N - mx); i < _n; i++)
    if (i + z[i] == N) {
      ans = z[i];
      break;
    }
  if (ans == -1) {
    cout << "Just a legend" << endl;
    return 0;
  }
  cout << s.substr(0, ans) << endl;
  return 0;
}
