import java .util.Scanner;
    public class hello{
        public static void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        String ori = sc.nextLine();
        String dup = sc.nextLine();
        int a = ori.length();
        int b = dup.length();
        int diff = 0;
       for(int i=0;i<a;i++){
           if(ori.charAt(i)!=dup.charAt(i)){
               diff++;
           }
       }
       if(diff>1){
            System.out.print(dup.charAt(i));
                    return;
       }
        if(b == a+1){
            for(int i=0;i<a;i++){
                if(dup.charAt(i)!= ori.charAt(i)){
                    System.out.print(dup.charAt(i));
                   
                    return;
                }
            }
            System.out.print(dup.charAt(b-1));
        }
        else{
            System.out.print("Invalid input");
        }
        }
        
    }