def hd(s1,s2):
	if len(s1) !=len(s2):
		raise ValueError("String not Equal")
		print (s1,s2)
	return sum(c1!=c2 for c1,c2 in zip(s1,s2))




print hd("muthur","kuthut")
print hd("muthur","kut") # for error
