X = input()
Y = input()
x = len(X)
y = len(Y)
if x == y or x == y + 1:
    Ans = ""
    for i in range(x):
        if i < x:
            Ans += X[i]
        if i < y:
            Ans += Y[i]
    print(Ans)
else:
    print("?")