import java.util.*;

public class Main{
    
    public static void main(String[] args){
        
        Scanner sc = new Scanner(System.in);
        if(!sc.hasNext()){
            System.out.println("Invalid input");
            return;
        }
        String s1 = sc.next();
        if(!sc.hasNext()){
            System.out.println("Invalid input");
            return;
        }
        String s2 = sc.next();
        if(s2.length()!=s1.length + 1){
            System.out.println("Invalid input");
            return;
        }
        
        int sum1=0, sum2=0;
        for(char c : s1.toCharArray()){
            sum1+=c;
        }
        for(char c : s2.toCharArray()){
            sum2+=c;
        }
        System.out.println((char)(sum2 - sum1));
    }
}