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>
#include<string.h>
#include<ctype.h>

int main(void){
    int K,answer = 0;
    scanf("%d",&K);

    for(int i=2; i<=K; i+=2){
        for(int j=1; j<=K; j+=2){
            answer++;
        }
    }

    printf("%d\n",answer);
    return 0;
}