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 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

unsigned int calc_combination_num(unsigned int N, int K);

int main(void)
{
    int K;
    unsigned int N;
    long long combination;

    scanf("%u %u", &N, &K);

    combination = calc_combination_num(N, K);
    printf("%llu\n", combination);
   
    return 0;

}

unsigned int calc_combination_num(unsigned int N, int K)
{
    long long combination;
    unsigned int pattern;

    if(K == 0){
        return 0;
    }

    if(K % 2 == 1){
        pattern = N / K;
        combination = (long long)pattern * pattern * pattern;
        return combination;
    }else{
        pattern = (N + K / 2) / K;
        combination = (long long)pattern * pattern * pattern;
        pattern = N / K;
        combination += (long long)pattern * pattern * pattern;
        return combination;
    }
}