Source: Codewars
Given two integers a and b, which can be positive or negative, find the sum
of all the integers between and including them and return it. If the two
numbers are equal return a or b.
Note: a and b are not ordered!
# (a, b) => output (explanation)
(1, 0) => 1 (1 + 0 = 1)
(1, 2) => 3 (1 + 2 = 3)
(0, 1) => 1 (0 + 1 = 1)
(1, 1) => 1 (1 since both are same)
(-1, 0) => -1 (-1 + 0 = -1)
(-1, 2) => 2 (-1 + 0 + 1 + 2 = 2)