#include <bits/stdc++.h>
using namespace std;
int II() {
  int q;
  scanf("%d", &q);
  return q;
}
int mod1 = 1000000007;
int mod2 = 1000000009;
int base = 353;
long long power1[1000006], power2[1000006];
long long hash_value1[1000006];
long long hash_value2[1000006];
void pre() {
  power1[0] = 1;
  power2[0] = 1;
  for (int i = 1; i < 1000004; i++) {
    power1[i] = (power1[i - 1] * base) % mod1;
    power2[i] = (power2[i - 1] * base) % mod2;
  }
}
void compute_hash(string &s) {
  hash_value1[0] = 0;
  hash_value2[0] = 0;
  int N = (int)s.size();
  for (int i = 0; i < N; i++) {
    hash_value1[i + 1] =
        (hash_value1[i] + ((s[i] - 'a' + 1) * power1[i]) % mod1) % mod1;
    hash_value2[i + 1] =
        (hash_value2[i] + ((s[i] - 'a' + 1) * power2[i]) % mod2) % mod2;
  }
}
pair<int, int> find_hash(int x, int y, int N) {
  int temp1 = (hash_value1[y + 1] - hash_value1[x] + mod1) % mod1;
  temp1 = (temp1 * power1[N - x - 1]) % mod1;
  int temp2 = (hash_value2[y + 1] - hash_value2[x] + mod2) % mod2;
  temp2 = (temp2 * power2[N - x - 1]) % mod2;
  return {temp1, temp2};
}
void solve() {
  pre();
  string s;
  cin >> s;
  compute_hash(s);
  int N = (int)s.size();
  vector<int> pa;
  for (int i = 1; i < N; i++) {
    int len = N - i;
    pair<int, int> temp1 = find_hash(0, len - 1, N);
    pair<int, int> temp2 = find_hash(i, i + len - 1, N);
    if (temp1 == temp2) {
      pa.push_back(i);
    }
  }
  reverse(pa.begin(), pa.end());
  int left = 0, right = pa.size() - 1;
  string ans = "Just a legend";
  while (left <= right) {
    int mid = (left + right) / 2;
    int len = pa[mid];
    int len_len = N - len;
    pair<int, int> temp1 = find_hash(0, len_len - 1, N);
    bool bo = 0;
    for (int i = 1; i < len; i++) {
      pair<int, int> temp2 = find_hash(i, i + len_len - 1, N);
      if (temp1 == temp2) {
        bo = 1;
        break;
      }
    }
    if (bo == 1) {
      ans = s.substr(0, len_len);
      left = mid + 1;
    } else
      right = mid - 1;
  }
  cout << ans << endl;
}
int main() { solve(); }
