#include <bits/stdc++.h>
using namespace std;
signed main() {
  string s;
  cin >> s;
  long long n = s.size();
  long long ldp[n];
  long long rdp[n];
  memset(ldp, 0, sizeof(ldp));
  memset(rdp, 0, sizeof(rdp));
  long long i = 0;
  long long j = 1;
  while (j < n) {
    if (s[i] == s[j]) {
      ldp[j] = i + 1;
      i++;
      j++;
    } else {
      if (i == 0) {
        ldp[j] = 0;
        j++;
      } else {
        i = ldp[i - 1];
      }
    }
  }
  i = n - 1;
  j = n - 2;
  while (j >= 0) {
    if (s[i] == s[j]) {
      rdp[j] = n - i;
      i--;
      j--;
    } else {
      if (i == n - 1) {
        rdp[j] = 0;
        j--;
      } else {
        i = n - 1 - rdp[i + 1];
      }
    }
  }
  long long ans = 0;
  long long ansInd = 0;
  for (long long i = 1; i < n - 1; i++) {
    if (ldp[i] > 0) {
      long long ind = i - ldp[i] + 1;
      if (ldp[i] == rdp[ind]) {
        if (ldp[i] > ans) {
          ans = ldp[i];
          ansInd = ind;
        }
      }
    }
  }
  if (ans > 0) {
    for (long long i = 0; i < ans; i++) {
      cout << s[ansInd + i];
    }
    cout << "\n";
  } else {
    cout << "Just a legend\n";
  }
}
