Problem:
Score : 100 points

Problem Statement

Find the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive). The order does not matter.

Constraints

2\leq K\leq 100

K is an integer.

Input

Input is given from Standard Input in the following format:

K

Output

Print the number of ways to choose a pair of an even number and an odd number from the positive integers between 1 and K (inclusive).

Sample Input 1

3

Sample Output 1

2

Two pairs can be chosen: (2,1) and (2,3).

Sample Input 2

6

Sample Output 2

9

Sample Input 3

11

Sample Output 3

30

Sample Input 4

50

Sample Output 4

625

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

solve :: Int -> [(Int, Int)]
solve k = [(x, y) | x <- [1..k], y <- [1..k], x /= y, mod x 2 == 0, mod y 2 == 1]

main = do
    k <- readLn
    print $ length (solve k)