def distance(strand_a, strand_b):
   	val1 = list(strand_a)
   	val2 = list(strand_b)
	count = 0
    if len(val1) == len(val2):
		for i in range(len(val1)):
			if val1[i] != val2[i]:
				count = count + 1
		return count
	else:
		raise Exception("ValueError")
	print(val1)
	print(val2)
	print(count)

distance("ABCD","ABCD")