def fun(s):
    res = ""
    a, b = len(s[0]), len(s[1])
    for i in range(max(a, b)):
        for j in range(2):
            if i == len(s[j]):
                if j == 1 and a - b == 1:
                    continue
                return "?"
            res += s[j][i]
    return res


xy = [input() for _ in range(2)]
print(fun(xy))