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 JavaScript (node.js v5.12) (judge verdict: Accepted)

function Main(input){
  input = input.split("\n");
  inputStr = input[0].split(" ");
  const N = Number.parseInt(inputStr[0]);
  //対象数を算出
  var ans = 0;
  if( N%2 === 0){
    //偶数
    ans = N/2-1;
  } else {
    //奇数
    ans = Math.ceil(N/2)-1;
  }
  console.log(ans);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));