What's the maximum number of regions definable by N zig-zag lines, each of which consists of two parallel infinite half-lines joined by a straight line segment?

Here is an example of 2 zig-zag lines yield 12 regions at the most.

The input consists of a sequence of N (<= 10000), which is the number of the zig-zag lines, one per line.

For each N, you should output the number of the maximum regions.