#include <iostream>
#include <cstring>
using namespace std;

int main()
{
    char s[300];
    cin.getline( s , 300 ) ;
    int lung = strlen ( s ) ;
    int max1 = 1 ;
    char rasp [300] ;
    int i = lung / 2 ;
    char aux [300] , aux2[300] ;
    for ( i = 0 ; i <= lung - 2 ; ++i )
    {
        for ( int j = 2 ; j <= lung - j ; ++j )
        {
            strncpy( aux ,s + i , j ) ;
            aux[j] = NULL ;
            for ( int k = i + j ; k <= lung - j ; ++k )
            {
                strncpy( aux2 , s + k , j );
                aux2[j]= NULL ;
                if ( strcmp( aux , aux2 ) == 0 )
                {
                    if ( j > max1 )
                    {
                        max1 = j ;
                        strcpy( rasp , aux ) ;
                    }
                    else
                    {
                        if ( j == max1 )
                        {
                            if ( strcmp( aux , rasp ) < 0 ) strcpy( rasp , aux) ;
                        }
                    }
                    break ;
                }
            }
        }
    }
    cout << rasp ;
    return 0;
}