Program to find Nth Hexagonal Number

Given an integer n, the task is to find the nth hexagonal number . The nth hexagonal number Hn is the number of distinct dots in a pattern of dots consisting of the outlines of regular hexagons with sides up to n dots when the hexagons are overlaid so that they share one vertex.

Input: n = 2
Output: 6

Input: n = 5
Output: 45

Input: n = 7
Output: 91

In general, a polygonal number (triangular number, square number, etc) is a number represented as dots or pebbles arranged in the shape of a regular polygon. The first few pentagonal numbers are 1, 5, 12, etc. 
If s is the number of sides in a polygon, the formula for the nth s-gonal number P (s, n) is 

nth s-gonal number P(s, n) = (s - 2)n(n-1)/2 + n

If we put s = 6, we get

n'th Hexagonal number H_n = 2(n*n)-n
= n(2n - 1)

Output: 

10th Hexagonal Number is = 190

Time complexity: O(1) since performing constant operations

Auxiliary space: O(1) since it is using constant variables