#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int maxn = 1e6 + 5;
int dp[maxn];
char t[maxn];
bool ha[maxn];
void cal(char *s) {
  int len = strlen(s);
  memset(dp, -1, sizeof(dp));
  for (int i = 1; i < len; ++i) {
    int j = dp[i - 1];
    while (j != -1 && s[j + 1] != s[i]) j = dp[j];
    if (s[j + 1] == s[i]) dp[i] = j + 1;
  }
}
int main() {
  scanf("%s", t);
  cal(t);
  int n = strlen(t);
  for (int i = 1; i < n - 1; ++i) {
    ha[dp[i] + 1] = 1;
  }
  int x = n - 1;
  while (x != -1 && !ha[dp[x] + 1]) x = dp[x];
  if (x == -1 || dp[x] == -1) return puts("Just a legend"), 0;
  for (int i = 0; i <= dp[x]; ++i) {
    printf("%c", t[i]);
  }
  puts("");
  return 0;
}
