#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:1000000000,1000000000")
using namespace std;
const long long inf = 1e18 + 7;
const long long mod = 1e9 + 7;
const double eps = 1e-9;
const double PI = 2 * acos(0.0);
const double E = 2.71828;
char s[1000005];
long long p[1000005];
int main(void) {
  gets(s);
  long long n = strlen(s), k = 0;
  for (long long i = 1; i < n; i++) {
    while (k && s[i] != s[k]) k = p[k - 1];
    if (s[i] == s[k]) k++;
    p[i] = k;
  }
  long long ans = p[n - 1];
  bool ok = 0;
  if (!ans) {
    puts("Just a legend");
    exit(0);
  }
  for (long long(i) = 0; (i) < (long long)(n - 1); (i)++)
    if (p[i] == ans) ok = 1;
  if (!ok) ans = p[ans - 1];
  if (ans <= 0) {
    puts("Just a legend");
    exit(0);
  }
  for (long long i = 0; i < ans; i++) {
    putchar(s[i]);
  }
  printf("\n");
  return 0;
}
