Dividing a Plane with Lines

Problem Background

Special for beginners, ^_^

Problem Description

There are $n\ (n \le 500)$ straight lines in the same plane. It is known that $p\ (p \ge 2)$ of them intersect at the same point. What is the maximum number of different regions these $n$ lines can divide the plane into?

Input Format

Two integers $n\ (n \le 500)$ and $p\ (2 \le p \le n)$.

Output Format

One positive integer, the maximum number of regions.

Sample Input

12 5

Sample Output

73

Constraints

1 second, 1024 KiB for each test case.