Sum of Even Numbers

Given an integer N, your task is to compute the sum of all even numbers between 1 and N (inclusive). If the value of N is out of the allowed range, output the error message N must be between 1 and 100 (inclusive)..

The problem can be mathematically formulated as follows:

$$ S = \sum_{k=1}^{\lfloor N/2 \rfloor} (2k) $$

inputFormat

Input is read from standard input (stdin). The input consists of a single integer N where 1 ≤ N ≤ 100.

outputFormat

Output to standard output (stdout) a single integer representing the sum of all even numbers from 1 to N (inclusive). If N is not in the allowed range, output the message N must be between 1 and 100 (inclusive)..

10

30