X = input()
Y = input()

if len(X) != len(Y) and len(X) != len(Y) + 1:
	print('?')
else:
	for i in range(len(X) + len(Y)):
		if i % 2 == 0:
			print(X[i//2],end = "")
		else:
			print(Y[i//2],end = "")