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: Time Limit Exceeded)

#include <stdio.h>

int main()
{
    int num, i = 1, j = 2, u = 0;
    
    scanf ("%d", &num);
    
    while (1){
        if (i % 2 == 0 && j % 2 != 0){
            u ++;
        }
        if (i % 2 != 0 && j % 2 == 0){
            u ++;
        }
        if (j == num){
        i ++;
        j = i + 1;
        }
        else
        j ++;
        if (i == (num - 1))
        break;
    }
    
    printf ("%d", u + 1);
    
    return 0;
}