#include <bits/stdc++.h>
using namespace std;
void solve() {
  string s;
  cin >> s;
  int n = s.length();
  int f[n], len = 0, i = 1, c = 0, j = 0;
  f[0] = 0;
  while (i < n) {
    if (s[len] == s[i])
      f[i++] = ++len;
    else {
      if (len != 0)
        len = f[len - 1];
      else
        f[i++] = 0;
    }
    if (i >= n) break;
  }
  string pattern = s.substr(0, f[n - 1]);
  int m = f[n - 1];
  i = 0, j = 0;
  int l = 0, r = m, mid;
  while (l < r) {
    mid = l + (r - l + 1) / 2;
    pattern = s.substr(0, mid);
    c = 0, i = 0, j = 0;
    while (i < n) {
      if (pattern[j] == s[i])
        i++, j++;
      else {
        if (j != 0)
          j = f[j - 1];
        else
          i++;
      }
      if (j == mid) {
        c++, j = f[j - 1];
      }
      if (i >= n) break;
    }
    if (c >= 3)
      l = mid;
    else
      r = mid - 1;
  }
  pattern = s.substr(0, l);
  reverse(s.begin(), s.end());
  string p2 = s.substr(0, l);
  reverse(p2.begin(), p2.end());
  if (l > 0 && pattern == p2)
    cout << pattern << endl;
  else
    cout << "Just a legend" << endl;
}
int main() {
  ios::sync_with_stdio(false);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
