Problem:
Score : 100 points

Problem Statement

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

1 \leq N \leq 10^6

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

4

Sample Output 1

1

There is only one way to choose two distinct integers totaling 4: to choose 1 and 3. (Choosing 3 and 1 is not considered different from this.)

Sample Input 2

999999

Sample Output 2

499999

Solution in PHP7 (7.0.15) (judge verdict: Accepted)

<?php
//error_reporting(0);
$N = trim(fgets(STDIN));
//list($A,$B) =  explode(" ",trim(fgets(STDIN)));
//$l = explode(" ",trim(fgets(STDIN)));


$v = intdiv($N,2)+1;
$res=0;

for($i=1;$i<$v;$i++) {
  if ($N-$i != $i) {
    $res++;
  }
}
printf("%d\n",$res);