#include <bits/stdc++.h>
using namespace std;
long long F[1000001];
char str[1000001];
long long sL = 0;
vector<long long> pufix[1000001];
int main() {
  long long i, j;
  string s;
  long long lookfor = 1;
  long long precomputed;
  long long need;
  cin >> s;
  for (i = 0; i < s.length(); i++) {
    str[i + 1] = s[i];
  }
  sL = s.length();
  F[0] = 0;
  F[1] = 0;
  for (i = 2; i <= sL; i++) {
    if (str[i] == str[lookfor]) {
      F[i] = lookfor;
      lookfor++;
    } else {
      precomputed = lookfor - 1;
      while (precomputed != 0 && str[i] != str[precomputed + 1]) {
        precomputed = F[precomputed];
      }
      if (precomputed == 0) {
        if (str[i] == str[1]) {
          F[i] = 1;
          lookfor = 2;
        } else {
          F[i] = 0;
          lookfor = 1;
        }
      } else {
        F[i] = precomputed + 1;
        lookfor = F[i] + 1;
      }
    }
    pufix[F[i]].push_back(i);
  }
  need = F[sL];
  while (need != 0) {
    for (i = 0; i < pufix[need].size(); i++) {
      if (pufix[need][i] != sL) {
        for (j = 1; j <= need; j++) {
          printf("%c", str[j]);
        }
        printf("\n");
        return 0;
      }
    }
    need = F[need];
  }
  printf("Just a legend\n");
  return 0;
}
