X = input()
Y = input()

if len(X) == len(Y) or len(X) == len(Y) + 1:
    ans = ['a']*(len(X) + len(Y))
    i = 0
    for x in X:
        ans[i] = x
        i += 2
    i = 1
    for y in Y:
        ans[i] = y
        i += 2
    
    print(''.join(ans))

else:
    print('?')