#include<bits/stdc++.h>

using namespace std;

const int M = 1e6 + 10 ;

char s[M] ;

int lens ;

bool vis[M] ;

int NEXT[M] ;

void get_fail () {

	NEXT[0] = -1 ;

	for (int i = 1 , at = -1  ; i < lens ; i ++) {

		while (at != -1 && s[at+1] != s[i]) at = NEXT[at] ;

		NEXT[i] = s[at+1] == s[i] ? ++at : at ;

	}

}

void solve () {

	get_fail () ;

	int at = NEXT[lens-1] ;

	while (at != -1) {

		vis[at] = 1 ;

		at = NEXT[at] ;

	}

	int maxn = -1 ;

	for (int i = lens-2 ; i >= 0 ; i --) {

		if (vis[NEXT[i]]) {

			if (NEXT[i] > maxn) maxn = NEXT[i] ;

		}

	}

	if (maxn == -1) puts ("Just a legend") ;

	for (int i = 0 ; i <= maxn ; i ++) printf ("%c" , s[i]) ; puts ("") ;

}

int main () {

	scanf ("%s" , s) ;

	lens = strlen (s) ;

	solve () ;

	return 0 ;

}