#include <iostream>
#include <algorithm>
#include <string>
#define ulli unsigned long long int
#define optimizar_io ios_base::sync_with_stdio(0);cin.tie(0);
using namespace std;

int n, p;
int P[1000002];
ulli H[1000002], b[1000002];
string A;

ulli hashing( int i, int j ){
	if( i == 0 )
		return H[j];
	return H[j] - ( H[i - 1] * b[j - i + 1] );
}

bool possible( int l ){
	for( int i = 1; i + l < n - 1; i++ )
		if( H[l] == hashing( i, i + l ) )
			return true;
	return false;
}

int main(){

	optimizar_io
	cin >> A;
	n = A.size();

	b[0] = 1; b[1] = 29;
	for( int i = 2; i <= n; i++ )
		b[i] = b[i - 1] * b[1];

	H[0] = ( A[0] - 'a' );
	for( int i = 1; i < n; i++ )
		H[i] = H[i - 1] * b[1] + ( A[i] - 'a' );

	for( int i = 0; i < n; i++ )
		if( hashing( 0, i ) == hashing( n - i - 1, n - 1 ) )
			P[++p] = i;

	int b = 1, e = p, m;

	if( !possible( P[1] ) ){
		cout << "Just a legend\n";
		return 0;
	}

	while( b != e ){
		m = ( b + e ) / 2 + 1;
		if( possible( P[m] ) )
			b = m;
		else
			e = m - 1;
	}

	for( int i = 0; i <= P[b]; i++ )
		cout << A[i];
	cout << "\n";


	return 0;

}