X = input().strip()
Y = input().strip()

if not (len(X) == len(Y) or len(X) == len(Y) + 1):
    print("?")
else:
    S = ""
    for i in range(len(Y)):
        S += X[i] + Y[i]
    if len(X) == len(Y) + 1:
        S += X[-1]
    print(S)