X = input()
Y = input()

if not (0 <= len(X) - len(Y) <= 1):
    print("?")
    exit()

elif(len(X) == len(Y)):
    for i in range(len(X)):
        print(X[i]+Y[i],end="")
    print()
else:
    for i in range(len(Y)):
        print(X[i]+Y[i],end="")
    print(X[-1])