#include <bits/stdc++.h>
using namespace std;
const long long inf = 1000000000000000000LL;
const double EPS = 1e-9;
const long long mod = 1000000007ll;
const int maxn = 500500;
int longestP[1000500];
char pat[1000500];
void KMP() {
  for (int i = 1, k = 0; pat[i]; i++) {
    while (k > 0 && pat[i] != pat[k]) k = longestP[k - 1];
    if (pat[i] == pat[k])
      longestP[i] = ++k;
    else
      longestP[i] = k;
  }
}
int main() {
  scanf("%s", pat);
  int n = strlen(pat);
  KMP();
  if (longestP[n - 1] == 0) {
    puts("Just a legend");
    return 0;
  }
  for (int i = 0; i + 1 < n; i++)
    if (longestP[n - 1] == longestP[i]) {
      for (int j = 0; j < longestP[i]; j++) putchar(pat[j]);
      return 0;
    }
  if (longestP[longestP[n - 1] - 1] == 0) {
    puts("Just a legend");
    return 0;
  }
  for (int j = 0; j < longestP[longestP[n - 1] - 1]; j++) putchar(pat[j]);
}
