#include <bits/stdc++.h>
using namespace std;
vector<unsigned long long> p_pow, h;
void p_hash(const string& str) {
  p_pow.resize(str.size());
  h.resize(str.size());
  const int p = 53;
  p_pow[0] = p;
  for (size_t i = 1; i < str.size(); ++i) p_pow[i] = p_pow[i - 1] * p;
  for (size_t i = 0; i < str.size(); ++i) {
    h[i] = (str[i] - 'a' + 1) * p_pow[i];
    if (i) h[i] += h[i - 1];
  }
}
unsigned long long s_hash(int i, int len) {
  unsigned long long res = h[i + len - 1];
  if (i) res -= h[i - 1];
  return res * p_pow[h.size() - i - 1];
}
int foo(int q) {
  int l = 0, r = h.size() - q, mid;
  while (r - l > 1) {
    mid = (l + r) / 2;
    if (s_hash(0, mid) == s_hash(q, mid))
      l = mid;
    else
      r = mid;
  }
  return l;
}
int main(void) {
  string str;
  cin >> str;
  p_hash(str);
  int mx = 0;
  for (int i = 1; i < str.size(); ++i) mx = max(mx, foo(i));
  for (int i = mx; i > 0; --i) {
    if (s_hash(0, i) == s_hash(str.size() - i, i)) {
      cout << str.substr(0, i);
      return 0;
    }
  }
  cout << "Just a legend";
  return 0;
}
