#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;
}