Number of Regions

You are given two integers $A$ and $B$. For every integer pair $(a, b)$ with $0 \le a < A$ and $0 \le b < B$, draw the line $y = ax + b$ on the plane. Different pairs give different lines, so $A \times B$ lines are drawn.

Write a program that finds how many regions those lines cut the plane into.

The first line contains $A$ and $B$. ($1 \le A, B \le 1200$)

Print the number of regions on the first line. This value goes past the 32-bit integer range, so use a 64-bit integer type.