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 (Clang 3.8.0) (judge verdict: Accepted)

#include <stdio.h>

int main(void){
    int K,i,j,n;
    n=0;
    
    scanf("%d",&K);
    
    for(i=1; i<=K; i++){
        for(j=i; j<=K; j++){
            if((i+j)%2==1)
                n++;
        }        
    }
    printf("%d\n",n);
}