title: How many subrectangle exists on a m x n grid
body: <p>Given a <code>m x n</code> grid, how many unique sub-rectangles exist on such a grid?</p>
  
  <p>For example, </p>
  
  <p><code>1 x 1</code> grid has 1 sub-rectangle.</p>
  
  <p><code>1 x 2</code> grid has 3 sub-rectangles.</p>
  
  <p>I am looking for a general formula that can be used to directly compute the number existing sub-rectangle.</p>