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: Wrong Answer)

k = gets.to_i
a = []
for i in 1..k do
  a << i
end
p a.combination(2).to_a.select {|as| (as[0]%2 == 0 && as[1]%2 != 0) || (as[0]%2 != 0 && as[1]%2 == 0)}