#include<iostream>
#include<string>
#include<cctype>
using namespace std;
int main (){
    string W1,W2,res="";
    if (!(cin>>W1>>W2))return 0;
    for (int i=0;i<max(W1.size());++i)
    {if(i<W2.size()){
        
    } (!isalpha(W1[i])){cout<<"Invalid input";return 0;}
    res+=W1[i];
}if(i<W2.size()){
     (!isalpha(W2[i])){cout<<"Invalid input";return 0;}
    res+=W2[i];
}
}
cout<<res;
return 0;
}