#include <bits/stdc++.h>
using namespace std;
using ld = long double;
using tint = long long;
using vi = vector<int>;
using pi = pair<int, int>;
void setIO(string name = "") {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
}
const int MOD = 1e9 + 7;
const int MX = 2e5 + 5;
const int INF = 1e9;
const ld PI = acos((ld)-1);
vector<int> z(string &second) {
  int n = second.size();
  vi z(n);
  for (int i = 1, l = 0, r = 0; i < n; ++i) {
    if (i <= r) z[i] = min(r - i + 1, z[i - l]);
    while (i + z[i] < n && second[z[i]] == second[i + z[i]]) ++z[i];
    if (i + z[i] - 1 > r) l = i, r = i + z[i] - 1;
  }
  return z;
}
int main() {
  setIO();
  string second;
  cin >> second;
  vi v = z(second);
  int maxi = 0, ret = 0;
  for (int i = int(0); i < int((int)(second).size()); i++) {
    if (v[i] + i == (int)(second).size() && maxi >= v[i]) {
      ret = v[i];
      break;
    }
    maxi = max(maxi, v[i]);
  }
  if (ret == 0)
    cout << "Just a legend" << endl;
  else
    cout << second.substr(0, ret) << endl;
}
