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 C++14 (GCC 5.4.1) (judge verdict: Accepted)

// This file is a "Hello, world!" in C++ language by GCC for wandbox.
#include <iostream>
#include <cstdlib>

int main()
{
    int k, ans;
    std::cin >> k;
    if(k%2 == 1){
        ans = ((k/2) + 1) * (k/2);
    }
    else {
        ans = (k/2) * (k/2);
    }
    std::cout << ans << std::endl;
}

// GCC reference:
//   https://gcc.gnu.org/

// C++ language references:
//   https://msdn.microsoft.com/library/3bstk3k5.aspx
//   http://www.cplusplus.com/
//   https://isocpp.org/
//   http://www.open-std.org/jtc1/sc22/wg21/

// Boost libraries references:
//   http://www.boost.org/doc/