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: Accepted)

#include<stdio.h>
int main(void)
{
    int k,i,j,s;
    scanf("%d",&k);
    for(j=2,s=0;j<=k;j++)
    {
        for(i=1;i<=j;i++)
        {
            if((i%2!=0 && j%2==0) || (i%2==0 && j%2!=0))
            {
                s++;
            }
        }
    }
    printf("%d\n",s);
    return 0;
}