import java.util.Scanner;
public class main{
    public static void main(String arg[]){
        Scanner s = new Scanner(System.in);
        String in1 = s.nextLine();
        String in2 = s.nextLine();
        int val=0;
        String out="";
        int i,check = 0;
        if(in1.substring(0,1) == in2.substring(0,1) || in1.substring(1,2) == in2.substring(1,2)){
            System.out.print("l");
                return;
        }
            if(1 + in1.length() == in2.length()){
                for(i=0;i<in1.length();i++){
                    if(in1.substring(i,i+1) == in2.substring(i,i+1) && check == 0){
                    int vall = in2.length();
                    out=in2.substring(vall-1,vall);
                    System.out.print(out);
                    check =1;
                    break;
                    }
                    else if(in1.substring(i,i+1) != in2.substring(i,i+1) && check == 0){
                    out=in2.substring(i,i+1);
                    System.out.print(c);
                    check =1;
                    break;
                    }
                }
                
            }

            else{
                System.out.print("Invalid input");
                return;
            }

            
 
    }
}