#include <bits/stdc++.h>
using namespace std;
int main() {
  ios ::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
  string s;
  cin >> s;
  int n = s.length();
  vector<int> z(n, 0);
  multiset<int> z_;
  for (int i = 1, l = 0, r = 0; i < n; i++) {
    if (i > r) {
      l = r = i;
      while (r < n && s[r - l] == s[r]) {
        r++;
      }
      z[i] = r - l;
      r--;
    } else {
      int j = i - l;
      if (i + z[j] <= r) {
        z[i] = z[j];
      } else {
        l = i;
        while (s[r - l] == s[r] && r < n) {
          r++;
        }
        z[i] = r - l;
        r--;
      }
    }
    if (z[i]) {
      z_.insert(z[i]);
    }
  }
  int Ans = 0;
  for (int i = n - 1; i > 0; i--) {
    if (z_.empty()) {
      break;
    } else if (z[i]) {
      if (z[i] + i == n) {
        auto it = z_.find(z[i]);
        z_.erase(it);
        it = z_.lower_bound(z[i]);
        if (it != z_.end()) {
          Ans = max(Ans, z[i]);
        }
      }
    }
  }
  if (Ans) {
    for (int i = 0; i < Ans; i++) {
      cout << s[i];
    }
  } else {
    cout << "Just a legend";
  }
  cout << endl;
}
