#include<iostream>

using namespace std;
int main(){
	string X,Y;
	cin >> X >> Y;
	int N = X.size();
	int M = Y.size();
	if(!(N==M || N-1==M)){
		cout << "?" << endl;
		return 0;
	}
	string ans;
	for(int i=0;i<M;++i){
		ans.push_back(X[i]);
		ans.push_back(Y[i]);
	}
	if(N-1 == M){
		ans.push_back(X.back());
	}
	cout << ans << endl;
}