Problem: Maria participates in a bicycle race.

The speedway takes place on the shores of Lake Lucerne, just repeating its contour. As you know, the lake shore consists only of straight sections, directed to the north, south, east or west.

Let's introduce a system of coordinates, directing the Ox axis from west to east, and the Oy axis from south to north. As a starting position of the race the southernmost point of the track is selected (and if there are several such points, the most western among them). The participants start the race, moving to the north. At all straight sections of the track, the participants travel in one of the four directions (north, south, east or west) and change the direction of movement only in bends between the straight sections. The participants, of course, never turn back, that is, they do not change the direction of movement from north to south or from east to west (or vice versa).

Maria is still young, so she does not feel confident at some turns. Namely, Maria feels insecure if at a failed or untimely turn, she gets into the water. In other words, Maria considers the turn dangerous if she immediately gets into the water if it is ignored.

Help Maria get ready for the competition — determine the number of dangerous turns on the track.

Input

The first line of the input contains an integer n (4 ≤ n ≤ 1000) — the number of straight sections of the track.

The following (n + 1)-th line contains pairs of integers (xi, yi) ( - 10 000 ≤ xi, yi ≤ 10 000). The first of these points is the starting position. The i-th straight section of the track begins at the point (xi, yi) and ends at the point (xi + 1, yi + 1).

It is guaranteed that:

  * the first straight section is directed to the north; 
  * the southernmost (and if there are several, then the most western of among them) point of the track is the first point; 
  * the last point coincides with the first one (i.e., the start position); 
  * any pair of straight sections of the track has no shared points (except for the neighboring ones, they share exactly one point); 
  * no pair of points (except for the first and last one) is the same; 
  * no two adjacent straight sections are directed in the same direction or in opposite directions. 

Output

Print a single integer — the number of dangerous turns on the track.

Examples

Input

6
0 0
0 1
1 1
1 2
2 2
2 0
0 0


Output

1


Input

16
1 1
1 5
3 5
3 7
2 7
2 9
6 9
6 7
5 7
5 3
4 3
4 4
3 4
3 2
5 2
5 1
1 1


Output

6

Note

The first sample corresponds to the picture:

<image>

The picture shows that you can get in the water under unfortunate circumstances only at turn at the point (1, 1). Thus, the answer is 1.

Solution (in JAVA):

Solution:

import java.io.*;
import java.lang.reflect.Array;
import java.math.BigInteger;
import java.util.*;
import java.util.Random;
import java.util.StringTokenizer;

public class Main {

    final long b = 31;
    String fileName = "";


    //////////////////////    SOLUTION   SOLUTION  SOLUTION    //////////////////////////////
    int INF = Integer.MAX_VALUE/10;
    long MODULO = 1000_1000_100l;
    int MAX_VALUE = 1000_1000;
    HashSet<Integer>[] graph;
    int[] color;
    boolean cycle = false;
    ArrayList<Integer> topSort = new ArrayList<>();
    void solve() throws IOException {
        int n = readInt();
        int MAX = 20000+400;
        int[] x = new int[n];
        int[] y = new int[n];
        x[0] = readInt() + 10100;
        y[0] = readInt() + 10100;
        for (int q=1; q<n; ++q){
            x[q] = readInt() + 10100;
            y[q] = readInt() + 10100;
        }
        int ans = 0;
        for (int i=0; i<n; ++i){
            int x1 = x[i];
            int y1 = y[i];
            int x2 = x[(i+1)%n];
            int y2 = y[(i+1)%n];
            int x3 = x[(i+2)%n];
            int y3 = y[(i+2)%n];

            int diffX1 = x2 - x1;
            int diffY1 = y2 - y1;

            int diffX2 = x3 - x2;
            int diffY2 = y3 - y2;

            if (diffX1 > 0 && diffY2 > 0) ans++;
            if (diffX1 < 0 && diffY2 < 0) ans++;
            if (diffY1 > 0 && diffX2 < 0) ans++;
            if (diffY1 < 0 && diffX2 > 0) ans++;

        }
        out.println(ans);
    }



    class Number implements Comparable<Number>{
        int x, cost;
        Number(int x, int cost){
            this.x = x;
            this.cost = cost;
        }

        @Override
        public int compareTo(Number o) {
            return Integer.compare(this.cost, o.cost);
        }
    }
    class Point{
        int x, y;
        Point(int x, int y){
            this.x = x;
            this.y = y;
        }
    }

    class Vertex implements Comparable<Vertex>{
        int num, depth, e, c;
        Vertex(int num, int depth, int e, int c){
            this.num = num;
            this.e = e;
            this.depth = depth;
            this.c = c;
        }

        @Override
        public int compareTo(Vertex o) {
            return Integer.compare(this.e, o.e);
        }
    }
    ///////////////////////////////////////////////////////////////////////////////////////////

    class Edge{
        int from, to, num;
        Edge(int to, int num){
            this.to = to;
            this.num = num;
        }
    }

    class SparseTable{
        int[][] rmq;
        int[] logTable;
        int n;
        SparseTable(int[] a){
            n = a.length;
            logTable = new int[n+1];
            for(int i = 2; i <= n; ++i){
                logTable[i] = logTable[i >> 1] + 1;
            }
            rmq = new int[logTable[n] + 1][n];
            for(int i=0; i<n; ++i){
                rmq[0][i] = a[i];
            }

            for(int k=1; (1 << k) < n; ++k){
                for(int i=0; i + (1 << k) <= n; ++i){
                    int max1 = rmq[k - 1][i];
                    int max2 = rmq[k-1][i + (1 << (k-1))];
                    rmq[k][i] = Math.max(max1, max2);
                }
            }
        }

        int max(int l, int r){
            int k = logTable[r - l];
            int max1 = rmq[k][l];
            int max2 = rmq[k][r - (1 << k) + 1];
            return Math.max(max1, max2);
        }
    }
    long checkBit(long mask, int bit){
        return (mask >> bit) & 1;
    }
    class Dsu{
        int[] parent;
        int countSets;
        Dsu(int n){
            countSets = n;
            parent = new int[n];
            for(int i=0; i<n; ++i){
                parent[i] = i;
            }
        }
        int findSet(int a){
            if(parent[a] == a) return a;
            parent[a] = findSet(parent[a]);
            return parent[a];
        }
        void unionSets(int a, int b){
            a = findSet(a);
            b = findSet(b);
            if(a!=b){
                countSets--;
                parent[a] = b;
            }
        }
    }
    static int checkBit(int mask, int bit) {
        return (mask >> bit) & 1;
    }
    boolean isLower(char c){
        return c >= 'a' && c <= 'z';
    }

    ////////////////////////////////////////////////////////////

    class SegmentTree{
        int[] t;
        int n;
        SegmentTree(int n){
            t = new int[4*n];
            build(new int[n+1], 1, 1, n);
        }
        void build (int a[], int v, int tl, int tr) {
            if (tl == tr)
                t[v] = a[tl];
            else {
                int tm = (tl + tr) / 2;
                build (a, v*2, tl, tm);
                build (a, v*2+1, tm+1, tr);
            }
        }

        void update (int v, int tl, int tr, int l, int r, int add) {
            if (l > r)
                return;
            if (l == tl && tr == r)
                t[v] += add;
            else {
                int tm = (tl + tr) / 2;
                update (v*2, tl, tm, l, Math.min(r,tm), add);
                update (v*2+1, tm+1, tr, Math.max(l,tm+1), r, add);
            }
        }

        int get (int v, int tl, int tr, int pos) {
            if (tl == tr)
                return t[v];
            int tm = (tl + tr) / 2;
            if (pos <= tm)
                return t[v] + get (v*2, tl, tm, pos);
            else
                return t[v] + get (v*2+1, tm+1, tr, pos);
        }
    }
    class Fenwik {
        long[] t;
        int length;

        Fenwik(int[] a) {
            length = a.length + 100;
            t = new long[length];

            for (int i = 0; i < a.length; ++i) {
                inc(i, a[i]);
            }
        }

        void inc(int ind, int delta) {
            for (; ind < length; ind = ind | (ind + 1)) {
                t[ind] = Math.max(delta, t[ind]);
            }
        }

        long getMax(int r) {
            long sum = 0;
            for (; r >= 0; r = (r & (r + 1)) - 1) {
                sum = Math.max(sum, t[r]);
            }
            return sum;
        }
    }
    int gcd(int a, int b){
        return b == 0 ? a : gcd(b, a%b);
    }
    long gcd(long a, long b){
        return b == 0 ? a : gcd(b, a%b);
    }
    double binPow(double a, int pow){
        if (pow == 0) return 1;
        if (pow % 2 == 1) {
            return a * binPow(a, pow - 1);
        } else {
            double c = binPow(a, pow / 2);
            return c * c;
        }
    }
    long binPow(long a, long b, long m) {
        if (b == 0) {
            return 1;
        }
        if (b % 2 == 1) {
            return ((a % m) * (binPow(a, b - 1, m) % m)) % m;
        } else {
            long c = binPow(a, b / 2, m);
            return (c * c) % m;
        }

    }
    int minInt(int... values) {
        int min = Integer.MAX_VALUE;
        for (int value : values) min = Math.min(min, value);
        return min;
    }

    int maxInt(int... values) {
        int max = Integer.MIN_VALUE;
        for (int value : values) max = Math.max(max, value);
        return max;
    }
    public static void main(String[] args) throws NumberFormatException, IOException {
        // TODO Auto-generated method stub
        new Main().run();
    }

    void run() throws NumberFormatException, IOException {
        solve();
        out.close();
    };

    BufferedReader in;
    PrintWriter out;

    StringTokenizer tok;
    String delim = " ";
    Random rnd = new Random();

    Main() throws FileNotFoundException {
        try {
            in = new BufferedReader(new FileReader("input.txt"));
            out = new PrintWriter("output.txt");
        } catch (Exception e) {
            if (fileName.isEmpty()) {
                in = new BufferedReader(new InputStreamReader(System.in));
                out = new PrintWriter(System.out);
            } else {
                in = new BufferedReader(new FileReader(fileName + ".in"));
                out = new PrintWriter(fileName + ".out");
            }

        }
        tok = new StringTokenizer("");
    }

    String readLine() throws IOException {
        return in.readLine();
    }

    String readString() throws IOException {
        while (!tok.hasMoreTokens()) {
            String nextLine = readLine();
            if (null == nextLine) {
                return null;
            }

            tok = new StringTokenizer(nextLine);
        }
        return tok.nextToken();
    }

    int readInt() throws NumberFormatException, IOException {
        return Integer.parseInt(readString());
    }
    byte readByte() throws NumberFormatException, IOException {
        return Byte.parseByte(readString());
    }
    int[] readIntArray (int n) throws NumberFormatException, IOException {
        int[] a = new int[n];
        for(int i=0; i<n; ++i){
            a[i] = readInt();
        }
        return a;
    }

    Integer[] readIntegerArray (int n) throws NumberFormatException, IOException {
        Integer[] a = new Integer[n];
        for(int i=0; i<n; ++i){
            a[i] = readInt();
        }
        return a;
    }

    long readLong() throws NumberFormatException, IOException {
        return Long.parseLong(readString());
    }

    double readDouble() throws NumberFormatException, IOException {
        return Double.parseDouble(readString());
    }
}