Sum of Even Numbers

Given a positive integer N, compute the sum of all even numbers from 1 to N (inclusive). In other words, calculate \(S = \sum_{\substack{i=1 \\ i \text{ is even}}}^{N} i\). The solution should be efficient to handle very large values of N.

Example: If N is 10, the even numbers are 2, 4, 6, and 8, so their sum is 30.

inputFormat

The input consists of a single line containing one integer N (1 ≤ N ≤ 10⁹).

outputFormat

Output the sum of all even numbers between 1 and N, inclusive.

10

30