A few years ago Sajjad left his school and register to another one due to security reasons. Now he wishes to find Amir, one of his schoolmates and good friends.

There are n schools numerated from 1 to n. One can travel between each pair of them, to do so, he needs to buy a ticket. The ticker between schools i and j costs <image> and can be used multiple times. Help Sajjad to find the minimum cost he needs to pay for tickets to visit all schools. He can start and finish in any school.

Input

The first line contains a single integer n (1 ≤ n ≤ 105) — the number of schools.

Output

Print single integer: the minimum cost of tickets needed to visit all schools.

Examples

Input

2


Output

0


Input

10


Output

4

Note

In the first example we can buy a ticket between the schools that costs <image>.
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.NoSuchElementException;

public class A {

  private void solve(FastScanner in, PrintWriter out) {
    int N = in.nextInt();
    if (N == 0) {
      out.println(0);
      return;
    }

    if (N % 2 == 0) {
      int bridges = N / 2 - 1;
      out.println(bridges);
    } else {
      int lands = N / 2;
      out.println(lands);
    }
  }

  public static void main(String[] args) {
    PrintWriter out = new PrintWriter(System.out);
    new A().solve(new FastScanner(), out);
    out.close();
  }

  class UnionFind {

    // par[i]：データiが属する木の親の番号。i == par[i]のとき、データiは木の根ノードである
    private int[] par;
    // sizes[i]：根ノードiの木に含まれるデータの数。iが根ノードでない場合は無意味な値となる
    private int[] sizes;

    // 木の数
    private int size;

    UnionFind(int n) {
      par = new int[n];
      sizes = new int[n];
      size = n;
      Arrays.fill(sizes, 1);
      // 最初は全てのデータiがグループiに存在するものとして初期化
      for (int i = 0; i < n; i++) {
        par[i] = i;
      }
    }

    /**
     * データxが属する木の根を得る
     */
    int find(int x) {
      if (x == par[x]) {
        return x;
      }
      return par[x] = find(par[x]);  // 根を張り替えながら再帰的に根ノードを探す
    }

    /**
     * 2つのデータx, yが属する木をマージする。
     * マージが必要なら true を返す
     */
    boolean unite(int x, int y) {
      // データの根ノードを得る
      x = find(x);
      y = find(y);

      // 既に同じ木に属しているならマージしない
      if (x == y) {
        return false;
      }

      // xの木がyの木より大きくなるようにする
      if (sizes[x] < sizes[y]) {
        int tx = x;
        x = y;
        y = tx;
      }

      // xがyの親になるように連結する
      par[y] = x;
      sizes[x] += sizes[y];
      sizes[y] = 0;  // sizes[y]は無意味な値となるので0を入れておいてもよい

      size--;
      return true;
    }

    /**
     * 2つのデータx, yが属する木が同じならtrueを返す
     */
    boolean isSame(int x, int y) {
      return find(x) == find(y);
    }

    /**
     * データxが含まれる木の大きさを返す
     */
    int partialSizeOf(int x) {
      return sizes[find(x)];
    }

    /**
     * 木の数を返す
     */
    int size() {
      return size;
    }
  }

  private static class FastScanner {

    private final InputStream in = System.in;
    private final byte[] buffer = new byte[1024];
    private int ptr = 0;
    private int bufferLength = 0;

    private boolean hasNextByte() {
      if (ptr < bufferLength) {
        return true;
      } else {
        ptr = 0;
        try {
          bufferLength = in.read(buffer);
        } catch (IOException e) {
          e.printStackTrace();
        }
        if (bufferLength <= 0) {
          return false;
        }
      }
      return true;
    }

    private int readByte() {
      if (hasNextByte()) {
        return buffer[ptr++];
      } else {
        return -1;
      }
    }

    private static boolean isPrintableChar(int c) {
      return 33 <= c && c <= 126;
    }

    private void skipUnprintable() {
      while (hasNextByte() && !isPrintableChar(buffer[ptr])) {
        ptr++;
      }
    }

    boolean hasNext() {
      skipUnprintable();
      return hasNextByte();
    }

    public String next() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      StringBuilder sb = new StringBuilder();
      int b = readByte();
      while (isPrintableChar(b)) {
        sb.appendCodePoint(b);
        b = readByte();
      }
      return sb.toString();
    }

    long nextLong() {
      if (!hasNext()) {
        throw new NoSuchElementException();
      }
      long n = 0;
      boolean minus = false;
      int b = readByte();
      if (b == '-') {
        minus = true;
        b = readByte();
      }
      if (b < '0' || '9' < b) {
        throw new NumberFormatException();
      }
      while (true) {
        if ('0' <= b && b <= '9') {
          n *= 10;
          n += b - '0';
        } else if (b == -1 || !isPrintableChar(b)) {
          return minus ? -n : n;
        } else {
          throw new NumberFormatException();
        }
        b = readByte();
      }
    }

    double nextDouble() {
      return Double.parseDouble(next());
    }

    double[] nextDoubleArray(int n) {
      double[] array = new double[n];
      for (int i = 0; i < n; i++) {
        array[i] = nextDouble();
      }
      return array;
    }

    double[][] nextDoubleMap(int n, int m) {
      double[][] map = new double[n][];
      for (int i = 0; i < n; i++) {
        map[i] = nextDoubleArray(m);
      }
      return map;
    }

    public int nextInt() {
      return (int) nextLong();
    }

    public int[] nextIntArray(int n) {
      int[] array = new int[n];
      for (int i = 0; i < n; i++) {
        array[i] = nextInt();
      }
      return array;
    }

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

    public String[] nextStringArray(int n) {
      String[] array = new String[n];
      for (int i = 0; i < n; i++) {
        array[i] = next();
      }
      return array;
    }

    public char[][] nextCharMap(int n) {
      char[][] array = new char[n][];
      for (int i = 0; i < n; i++) {
        array[i] = next().toCharArray();
      }
      return array;
    }

    public int[][] nextIntMap(int n, int m) {
      int[][] map = new int[n][];
      for (int i = 0; i < n; i++) {
        map[i] = nextIntArray(m);
      }
      return map;
    }
  }
}
