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 Ruby (2.3.3) (judge verdict: Accepted)

k = gets.to_i

m = (k / 2) ## 奇数 50 偶数 50

count = 0

if k % 2 == 0 
  count = m * m  ## 偶数
else
  count = m * (m + 1) ## 奇数
end

puts count.to_s  ## 文字列を出力