def internal_ord(c):
	c = c.lower()
	internal_ord = ord(c) - ord("a")
	if(internal_ord < 0 or internal_ord > 25):
		raise ValueError("'{}' is an unsupported character".format(c))
	return internal_ord

def internal_chr(i):
	return chr(ord("a") + i)


def encode_or_keeps_space(c):
	if(c == " "):
		return (False, c)
	return (True, internal_ord(c))

def prepare_string(s):
	return (encode_or_keeps_space(c) for c in s)


def _caesar(s, K):
	for encode, i in prepare_string(s):
		if(encode):
			yield internal_chr((i + K) % 26)
		else:
			yield i

def caesar(s, K):
	return "".join(_caesar(s, K))