Count Distinct Rectangles in N*N Chessboard

Given a n x n Chessboard. The task is to count distinct rectangles from the chessboard. For example, if the input is 8 then the output should be 36.
Examples:

Input: n = 4
Output: 10

Input: n = 6
Output: 21

Suppose n = 8 i.e. 8 x 8 chessboard is given, So different rectangles that can be formed are: 

1 x 1, 1 x 2, 1 x 3, 1 x 4, 1 x 5, 1 x 6, 1 x 7, 1 x 8 = 8
2 x 2, 2 x 3, 2 x 4, 2 x 5, 2 x 6, 2 x 7, 2 x 8 = 7
3 x 3, 3 x 4, 3 x 5, 3 x 6, 2 x 7, 3 x 8 = 6
4 x 4, 4 x 5, 4 x 6, 4 x 7, 4 x 8 = 5
5 x 5, 5 x 6, 5 x 7, 5 x 8 = 4
6 x 6, 6 x 7, 6 x 8 = 3
7 x 7, 7 x 8 = 2
8 x 8 = 1

So total distinct rectangles formed = 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 = 36. which is the sum of the first 8 natural numbers. So in general, distinct rectangles that can be formed in an n x n chessboard are: sum of the first n natural numbers = n*(n+1)/2

10

Time Complexity: O(1)
Auxiliary Space: O(1)