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;
  int i, j, k;
  int count = 0;

  scanf("%d", &K);

  for(i = 1; i <= K; i++){
    for(j = i+1; j <= K; j++){
      if(i%2 == 0 && j%2 == 1){
        count++;
      }else if(i%2 == 1 && j%2 == 0){
        count++;
      }
    }
  }

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