X = input()
Y = input()

N = len(X)
M = len(Y)

if N == M + 1 or N == M:
    ans = [""] * (N + M)
    for i in range(N):
        ans[2 * i] = X[i]
    for i in range(M):
        ans[2 * i + 1] = Y[i]

    print("".join(ans))
else:
    print("?")