from itertools import islice


def _load(filename):
	with open(filename) as f:
		for l in f:
			yield l.rstrip()


def load(filename):
	f = list(_load(filename))
	for j in range(0, int(len(f) / 3)):
		i = j*3
		yield f[i], f[i+1], f[i+2]


def overlap(c1, c2, c3):
	return list(set(c1).intersection(set(c2)).intersection(set(c3)))[0]

def priority(c):
	n = ord(c)

	if ord("a") <= n <= ord("z"):
		return n - ord("a") + 1
	if ord("A") <= n <= ord("Z"):
		return n - ord("A") + 27

	raise Exception()

def calcsum(l):
	l = [overlap(*x) for x in l]
	t = sum([priority(*x) for x in l])
	return t

assert priority("a") == 1
assert priority("z") == 26
assert priority("A") == 27
assert priority("Z") == 52

l = list(load("demo.txt"))
assert l[0] == ("vJrwpWtwJgWrhcsFMMfFFhFp", "jqHRNqRjqzjGDLGLrsFMfFZSrLrFZsSL", "PmmdzqPrVvPwwTWBwg")
assert l[1] == ("wMqvLMZHhHMvwLHjbvcjnnSBnvTQFn", "ttgJtRGJQctTZtZT", "CrZsJsPPZsGzwwsLwLmpwMDw")

assert overlap(*l[0]) == "r"
assert overlap(*l[1]) == "Z"

assert calcsum(l) == 70

l = list(load("data.txt"))
print("Sum is:", calcsum(l))