#include <bits/stdc++.h>
using namespace std;
#pragma comment(linker, "/STACK:64000000")
inline void imp() {
  printf("Just a legend\n");
  exit(0);
}
string s;
int n;
long long H[1 << 20];
long long PP[1 << 20];
inline long long getHash(int l, int r) {
  long long res = H[r];
  if (l) res -= H[l - 1] * PP[r - l + 1];
  return res;
}
inline int check(int l) {
  long long PH = getHash(0, l - 1);
  for (int i = 1; i + l < n; ++i) {
    long long curHash = getHash(i, i + l - 1);
    if (curHash == PH) return 1;
  }
  return 0;
}
int main() {
  cin >> s;
  n = (int)s.length();
  if (n < 3) imp();
  H[0] = (long long)s[0];
  for (int i = 1; i < n; ++i) H[i] = H[i - 1] * 1103 + (long long)s[i];
  PP[0] = 1;
  for (int i = 1; i <= n; ++i) PP[i] = PP[i - 1] * 1103;
  int res = -1;
  int l = 1, r = n - 2;
  while (l <= r) {
    int mid = (l + r) >> 1;
    if (check(mid)) {
      res = mid;
      l = mid + 1;
    } else {
      r = mid - 1;
    }
  }
  if (res == -1) imp();
  for (; res; --res) {
    if (getHash(0, res - 1) == getHash(n - res, n - 1)) break;
  }
  if (!res) res = -1;
  if (res != -1) {
    for (int i = 0; i < res; ++i) printf("%c", s[i]);
    printf("\n");
    return 0;
  }
  imp();
  return 0;
}
