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.

Examples

Input

4


Output

1


Input

999999


Output

499999
import java.util.*;
import java.io.*;
import java.util.function.*;

class Main {
    static Scanner scanner = new Scanner();
    static int mod = 1000000007;
    static long inf = 0x7ffffffffffffffL;

    public static void main(String[]$) throws Exception {
        int n = scanner.nextInt();
        System.out.println(n % 2 == 0 ? n / 2 - 1 : n / 2);
    }

    static class System {
        static PrintWriter out = new PrintWriter(java.lang.System.out);

        static {
            Runtime.getRuntime().addShutdownHook(new Thread(System.out::flush));
        }

        static void exit() {
            java.lang.System.exit(0);
        }
    }

    static class Scanner {
        BufferedReader reader = new BufferedReader(new InputStreamReader(java.lang.System.in), 0x10000);
        StringTokenizer tokenizer;

        String next() {
            try {
                while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                    tokenizer = new StringTokenizer(reader.readLine());
                }
            } catch (IOException ignored) {
            }
            return tokenizer.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        char[][] nextCharArray(int n, int m) {
            char[][] a = new char[n][m];
            for (int i = 0; i < n; i++) {
                a[i] = next().toCharArray();
            }
            return a;
        }
        int[] nextIntArray(int n) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextInt();
            }
            return a;
        }

        int[] nextIntArray(int n, IntUnaryOperator op) {
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = op.applyAsInt(nextInt());
            }
            return a;
        }

        long[] nextLongArray(int n) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) {
                a[i] = nextLong();
            }
            return a;
        }

        long[] nextLongArray(int n, LongUnaryOperator op) {
            long[] a = new long[n];
            for (int i = 0; i < n; i++) {
                a[i] = op.applyAsLong(nextLong());
            }
            return a;
        }
    }
}
