#include <bits/stdc++.h>
using namespace std;
const long long LINF = 1e18 + 100100, INF = 2e9 + 100100;
const double EPS = 1e-9, PI = acos(-1);
const int MOD = 1e9 + 7, P = 239, MAXN = 1e5 + 100;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
vector<int> zfunc(const string &s) {
  int n = (int)(s).size();
  vector<int> z(n);
  for (int i = 1, l = 0, r = 0; i < n; ++i) {
    if (i <= r) {
      z[i] = min(r - i, z[i - l]);
    }
    while (i + z[i] < n && s[z[i]] == s[i + z[i]]) {
      ++z[i];
    }
    if (i + z[i] > r) {
      l = i, r = i + z[i];
    }
  }
  return z;
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long double START_TIME = clock();
  string s;
  cin >> s;
  int n = (int)(s).size();
  vector<int> z = zfunc(s);
  int limit = -1;
  for (int i = 0; i < n; ++i) {
    if (i + z[i] < n) {
      limit = max(limit, z[i]);
    } else {
      limit = max(limit, z[i] - 1);
    }
  }
  for (int i = 0; i < n; ++i) {
    if (i + z[i] == n) {
      if (limit >= z[i]) {
        cout << s.substr(0, z[i]);
        return 0;
      }
    }
  }
  cout << "Just a legend";
  long double WORK_TIME = (clock() - START_TIME) / CLOCKS_PER_SEC;
  if (false) cerr << "\n[" << ("WORK_TIME") << "]: " << (WORK_TIME);
  ;
  return 0;
}
