#include <bits/stdc++.h>
using namespace std;
string str;
int l;
long long pre[1000001], mul[1000001];
bool check(int indx) {
  int l1 = indx + 1;
  long long val = pre[indx] * mul[l - 1];
  for (int i = 1; i < l - l1; i++) {
    long long val1 = pre[i + l1 - 1] - pre[i - 1];
    val1 *= mul[l - 1 - i];
    if (val1 == val) {
      return true;
    }
  }
  return false;
}
int main(void) {
  cin >> str;
  l = str.length();
  mul[0] = 1;
  pre[0] = str[0] - 'a' + 1;
  long long val = 131;
  for (int i = 1; i < l; i++) {
    pre[i] = val * (str[i] - 'a' + 1) + pre[i - 1];
    mul[i] = val;
    val *= 131;
  }
  vector<int> tracks;
  for (int i = 0; i < l - 1; i++) {
    long long val1 = pre[i] * mul[l - 1];
    long long val2 = pre[l - 1] - pre[l - i - 2];
    val2 *= mul[i];
    if (val1 == val2) {
      tracks.push_back(i);
    }
  }
  if (tracks.size() == 0) {
    cout << "Just a legend\n";
    return 0;
  }
  int l1 = 0, r1 = tracks.size() - 1;
  while (l1 < r1) {
    int mid = l1 + (r1 - l1 + 1) / 2;
    if (check(tracks[mid])) {
      l1 = mid;
    } else {
      r1 = mid - 1;
    }
  }
  if (check(tracks[l1])) {
    string tmp = str.substr(0, tracks[l1] + 1);
    cout << tmp << endl;
  } else {
    cout << "Just a legend\n";
  }
}
