Formula to calculate sum between two numbers

SumBetweenNums.py

# Reference : https://www.quora.com/How-do-you-calculate-the-sum-of-the-numbers-between-x-and-y

# This formula does't depened on which one of a or b is greater
def sumBetweenNums(a,b):
    m = abs(a-b+1)
    k = a+b
    r = (m*k)/2
    return r

# OR

# If a > b, else make the formula reverse
def sumBetweenNums(a, b):
    return ((a * (a + 1)) - (b * (b+1)) ) // 2