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 Go (1.6) (judge verdict: Accepted)

package main
 
import (
	"fmt"
)

func main() {
	var (
		a int
		o int
	)
	fmt.Scanf("%d", &a)
	if a % 2 == 0  {
		o = (a - 2) / 2
	} else {
		o = (a - 1) / 2
	}
	fmt.Print(o)
}