sgu 193

import java.io.BufferedInputStream;
import java.math.BigInteger;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner cin = new Scanner(new BufferedInputStream(System.in));
        BigInteger num, two = BigInteger.valueOf(2), one = BigInteger.ONE;
        while (cin.hasNext()) {
            num = cin.nextBigInteger();
            if (num.mod(two).compareTo(one) == 0) {
                num = num.subtract(one).divide(two);
            } else {
                num = num.divide(two).subtract(one);
                if (num.mod(two).compareTo(one) != 0) {
                    num = num.subtract(one);
                }
            }
            System.out.println(num);
        }
    }
}