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: Wrong Answer)

package main
 
import (
  "fmt"
)
 
func main(){
  var N int
  var count int
  fmt.Scan(&N)
  
  for i:=1;i<N;i++{
    var j = N - i
    if i == j{
      break
    }
    count++
  }
  fmt.Println(count)
}