#include <bits/stdc++.h>
using namespace std;
long long const INF = 2147483647, R_INF = 9223372036854775807, MOD = 1e9 + 7;
const int N = 1e6 + 1;
int max_border[N];
void prefix(string str) {
  max_border[0] = -1;
  for (int i = (1); i < (str.size()); i++) {
    max_border[i] = max_border[i - 1];
    while (~max_border[i] && str[i] != str[max_border[i] + 1])
      max_border[i] = max_border[max_border[i]];
    if (str[i] == str[max_border[i] + 1]) max_border[i]++;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  string str;
  cin >> str;
  prefix(str);
  int maxi1 = -1, maxi2 = max_border[str.size() - 1];
  for (int i = (1); i < (str.size() - 1); i++)
    maxi1 = max(maxi1, max_border[i]);
  if (maxi1 >= maxi2)
    maxi1 = maxi2;
  else {
    while (maxi1 < maxi2) {
      maxi2 = max_border[maxi2];
    }
  }
  maxi1 = maxi2;
  if (maxi1 == -1) return cout << "Just a legend", 0;
  for (int i = (0); i < (maxi1 + 1); i++) {
    cout << str[i];
  }
}
