#include <bits/stdc++.h>
using namespace std;
const char nl = '\n';
const int mod = 1000000007;
const double pi = 4 * atan(1);
const int inf = 2000000000;
const long long linf = (long long)1e18;
const int pr = 37;
const int maxn = 1000001;
const int maxn_big = 1000001;
string s;
vector<int> v;
unsigned long long h[maxn], p[maxn];
unsigned long long get_hash(int l, int r) {
  int len = (r - l) + 1;
  if (!l) return h[r];
  return h[r] - h[l - 1] * p[len];
}
bool check(int pf) {
  unsigned long long pref_hash = get_hash(0, pf);
  bool has_ans = 0;
  for (int i = 1; i < ((int)(s.size())) - 1; ++i) {
    int r = i + pf;
    if (r >= ((int)(s.size())) - 1) break;
    if (get_hash(i, r) == pref_hash) {
      has_ans = 1;
      break;
    }
  }
  return has_ans;
}
int main() {
  ios_base ::sync_with_stdio(0);
  cin.tie(0);
  srand(time(NULL));
  cin >> s;
  h[0] = s[0];
  p[0] = 1;
  for (int i = 1; i < ((int)(s.size())); ++i) {
    h[i] = h[i - 1] * pr + s[i];
    p[i] = p[i - 1] * pr;
  }
  for (int i = 0; i < ((int)(s.size())); ++i) {
    int l = ((int)(s.size())) - 1 - i;
    if (get_hash(0, i) == get_hash(l, ((int)(s.size())) - 1)) v.push_back(i);
  }
  if (!((int)(v.size()))) {
    cout << "Just a legend";
    return 0;
  }
  int l = -1, r = ((int)(v.size()));
  bool has = 0;
  while (r - l > 1) {
    int mid = (r + l) / 2;
    if (check(v[mid])) {
      l = mid;
      has = 1;
    } else {
      r = mid;
    }
  }
  if (!has) {
    cout << "Just a legend";
    return 0;
  }
  string res = "";
  for (int i = 0; i <= v[l]; ++i) res += s[i];
  cout << res;
  return 0;
}
