#include<stdio.h>
#include<string.h>
#include<ctype.h>
int main(){
    char s1[20];
    char s2[20];
    scanf("%s%s",s1,s2);
    if(isalpha(s1) || isalpha(s2){
        printf("Invalid input");
    }
    else{
        printf("%s",strcat(s1,s2));
    }
}