Sum of numbers in an interval X15128

Statement

Write a funcion @sum_interval(a, b)@ that receives two integers $a,b$ such that $a \le b$ , and returns the sum of all integers in $[a,b]$ .

Sample session

>>> sum_interval(5,10)
45
>>> sum_interval(1,100)
5050
>>> sum_interval(10,20)
165

Information