#include <bits/stdc++.h>
int main() {
  std::ios_base::sync_with_stdio(false);
  std::string s;
  std::cin >> s;
  int n = s.length();
  std::vector<int> prefix_fun(n, 0);
  for (int i = 1; i < n; i++) {
    int j = prefix_fun[i - 1];
    while (j > 0 && s[i] != s[j]) {
      j = prefix_fun[j - 1];
    }
    if (s[i] == s[j]) {
      j++;
    }
    prefix_fun[i] = j;
  }
  if (prefix_fun[n - 1] == 0) {
    std::cout << "Just a legend";
    return 0;
  }
  for (int i = 1; i < n - 1; i++) {
    if (prefix_fun[i] == prefix_fun[n - 1]) {
      std::cout << s.substr(0, prefix_fun[i]);
      return 0;
    }
  }
  int j = prefix_fun[prefix_fun[n - 1] - 1];
  if (j == 0) {
    std::cout << "Just a legend";
    return 0;
  }
  std::cout << s.substr(0, j);
  return 0;
}
