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)

def eve_odo(a, b)
    return true if a%2 == 0 and b%2 != 0
    return true if a%2 != 0 and b%2 == 0
    return false
end

def aa
    k = gets.chomp.to_i

    o = []
    for i in 1..(k-1)
        for j in (i+1)..k
            o.push([i, j]) if eve_odo(i, j)
        end
    end

    puts o.length
end

aa