#include <bits/stdc++.h>
using namespace std;
long long n, i, j, ans = 0;
string s;
long long lps[10000000];
void computelps(string s) {
  long long len = 0;
  lps[0] = 0;
  int i = 1;
  long long M = s.length();
  while (i < M) {
    if (s[i] == s[len])
      len++, lps[i] = len, i++;
    else {
      if (len != 0)
        len = lps[len - 1];
      else
        lps[i] = 0, i++;
    }
  }
}
bool kmp(string txt, string pat) {
  int i = 0;
  int j = 0;
  while (i < txt.length()) {
    if (pat[j] == txt[i]) {
      j++;
      i++;
    }
    if (j == pat.length()) return true;
    if (i < txt.length() && pat[j] != txt[i]) {
      if (j != 0)
        j = lps[j - 1];
      else
        i = i + 1;
    }
  }
  return false;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> s;
  string str = "Just a legend";
  computelps(s);
  n = s.length();
  if (n < 3) return cout << str, 0;
  string t = s;
  t.pop_back();
  t = t.substr(1);
  if (lps[n - 1] == 0 or s.length() <= 2) return cout << str, 0;
  for (long long k = lps[n - 1]; k > 0; k = lps[k - 1])
    if (kmp(t, s.substr(0, k))) return cout << s.substr(0, k), 0;
  cout << str;
}
