#include <iostream>
using namespace std;

int main() {
    string a,b,res = "";
    cin >> a >> b;
    
    for(char c : a) if (isalpha(c)){
        cout << "Invalid input"; 
        return 0;
    }
    for(char c : b) if (isalpha(c)){
        cout << "Invalid input";
        return 0;
    }
    int i = 0
    while(i < a.size() && i < b.size()))
        res += a[i] + string(1, b[i++]);
    res += a.substr(i) + b.substr(i);
    
    cout << res;
}