common_sub.java

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;
public class temp
{

public static void main(String[] args)
{
        String str=(new Scanner(System.in)).nextLine();
        if(str.length() == 1) {
            System.out.println(str);
            return;
        }
        System.out.println(longestSubstr(str));
}
public static String longestSubstr(String s)
{
        String result="";
        int len=s.length();
        for(int i=len/2; i>0; i--)
        {
                String front=s.substring(0,i);
                String end=s.substring(len-i);
                if(front.equals(end))
                {
                        result=front;
                        break;
                }
        }
        return result;
}
}