X = input()
Y = input()

if 0 <= len(X)-len(Y) <= 1:
    ans = []
    for i in range(len(X)+len(Y)):
        if i%2 == 0:
            ans.append(X[i//2])
        else:
            ans.append(Y[i//2])
    print("".join(ans))
else:
    print("?")