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 n int
    fmt.Scan(&n)
    if n % 2 == 0 {
        fmt.Println(n/2-1)
    } else {
        fmt.Println(n/2)
    }
}