#include <bits/stdc++.h>
using namespace std;
int Set(int N, int pos) { return N = N | (1 << pos); }
int reset(int N, int pos) { return N = N & ~(1 << pos); }
bool check(int N, int pos) { return (bool)(N & (1 << pos)); }
void CI(int &_x) { scanf("%d", &_x); }
void CO(int &_x) { cout << _x; }
template <typename T>
void getarray(T a[], int n) {
  for (int i = 0; i < n; i++) cin >> a[i];
}
template <typename T>
void printarray(T a[], int n) {
  for (int i = 0; i < n - 1; i++) cout << a[i] << " ";
  cout << a[n - 1] << endl;
}
const double EPS = 1e-9;
const int INF = 0x7f7f7f7f;
int dr8[8] = {1, -1, 0, 0, 1, -1, -1, 1};
int dc8[8] = {0, 0, -1, 1, 1, 1, -1, -1};
int dr4[4] = {0, 0, 1, -1};
int dc4[4] = {-1, 1, 0, 0};
int kn8r[8] = {1, 2, 2, 1, -1, -2, -2, -1};
int kn8c[8] = {2, 1, -1, -2, -2, -1, 1, 2};
char pattern[1000000 + 5];
int F[1000000 + 5];
void build_failure_function() {
  int m = strlen(pattern);
  F[0] = F[1] = 0;
  for (int i = 2; i <= m; i++) {
    int j = F[i - 1];
    for (;;) {
      if (pattern[j] == pattern[i - 1]) {
        F[i] = j + 1;
        break;
      }
      if (j == 0) {
        F[i] = 0;
        break;
      }
      j = F[j];
    }
  }
}
int main() {
  scanf("%s", &pattern);
  int len = strlen(pattern);
  build_failure_function();
  int l = F[len];
  if (l == 0) {
    puts("Just a legend");
    return 0;
  }
  for (int i = l + 1; i <= (len - 1); i++) {
    if (F[len] == F[i]) {
      for (int j = 0; j < l; j++) cout << pattern[j];
      puts("");
      return 0;
    }
  }
  if (F[F[len]] == 0) {
    puts("Just a legend");
  } else {
    l = F[F[len]];
    for (int j = 0; j < l; j++) cout << pattern[j];
    puts("");
  }
  return 0;
}
