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: Wrong Answer)

function main(input) {
  var s = input[0]-0;
  console.log(Math.ceil(s/2));
}

main(require('fs').readFileSync('/dev/stdin', 'utf8').split("\n"))