#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9 + 9;
const long long LINF = 1e12;
const int MAXN = 1e6 + 5;
const int MOD = 1e9 + 7;
const double eps = 1e-14;
int n;
int pf[MAXN];
string s;
void pref_func() {
  for (int i = 1; i < n; i++) {
    int j = pf[i - 1];
    while (j > 0 && s[i] != s[j]) j = pf[j - 1];
    if (s[i] == s[j]) j++;
    pf[i] = j;
  }
}
void solve() {
  cin >> s;
  n = s.size();
  pref_func();
  int k = 0;
  for (int i = 1; i < n - 1; i++) {
    if (pf[i] == pf[n - 1]) k = pf[n - 1];
  }
  for (int i = 1; i < n - 1; i++) {
    if (pf[i] == pf[pf[n - 1] - 1]) k = max(k, pf[pf[n - 1] - 1]);
  }
  if (k) {
    for (int i = 0; i < k; i++) cout << s[i];
  } else
    cout << "Just a legend";
}
int main() {
  if (!1) {
    freopen(
        "474"
        ".in",
        "r", stdin);
    freopen(
        "474"
        ".out",
        "w", stdout);
  }
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) solve();
  return 0;
}
