Find all the possible Rectangles(excluding squares) in a Grid

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.NET

algorithm

mathematics

Given a rectangle(or even a square of N x N) H x W, find all the possible rectangles (excluding squares) that can fit into H x W (or N x N).

My tutor spoon fed me this expression but left it up to us to figure out why and how it works.

for H = 1 to Height
   for W = 1 to Width
      totalRect += (H x W) - Math.min(H,W)
   next
next

The expression is absolutely beautiful in its simplicity and it works perfectly. But I cant figure out why or even how it works ( I can't derive it). Anyone out there not of a lesser mind that can help me figure this out?

Thanks