#include <bits/stdc++.h>
using namespace std;
inline int gc() {
  static const int BUF = 1e7;
  static char buf[BUF], *bg = buf + BUF, *ed = bg;
  if (bg == ed) fread(bg = buf, 1, BUF, stdin);
  return *bg++;
}
inline int ri() {
  int x = 0, f = 1, c = gc();
  for (; c < 48 || c > 57; f = c == '-' ? -1 : f, c = gc())
    ;
  for (; c > 47 && c < 58; x = x * 10 + c - 48, c = gc())
    ;
  return x * f;
}
const int maxN = 1e5 + 7;
string s;
void KMP_next(string &T, vector<int> &nxt) {
  nxt.resize(T.size() + 1);
  nxt[0] = -1;
  int t = -1, i = 0;
  while (i < T.size()) {
    (0 > t || T[i] == T[t]) ? nxt[++i] = ++t : t = nxt[t];
  }
}
int main() {
  while (cin >> s) {
    string ans = "";
    vector<int> nxt;
    KMP_next(s, nxt);
    unordered_set<int> si;
    for (int i = 0; i < (nxt.size() - 1); ++i) si.insert(nxt[i]);
    int p = nxt[nxt.size() - 1];
    while (p > 0) {
      if (si.find(p) != si.end()) {
        ans = s.substr(0, p);
        break;
      }
      p = nxt[p];
    }
    if (ans == "")
      cout << "Just a legend\n";
    else
      cout << ans << endl;
  }
  return 0;
}
