import java.util.*;

public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    }
} import java.util.*; util.* java public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    } main {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int[] p = new int[n]; p new int[n] n for (int i=0;i<n;i++)p[i]=sc.nextInt(); int i=0; int i=0; i 0 i<n i n i++ i++ i p[i]=sc.nextInt(); p[i]=sc.nextInt() p[i] p i sc.nextInt() sc.nextInt sc int ans = 0; ans 0 for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++; int i=1; int i=1; i 1 i<n-1 i n-1 n 1 i++ i++ i if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++; if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++; !((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])) ((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])) (p[i-1]>p[i]&&p[i+1]>p[i]) p[i-1]>p[i] p[i-1] p i-1 i 1 p[i] p i p[i+1]>p[i] p[i+1] p i+1 i 1 p[i] p i (p[i-1]<p[i]&&p[i+1]<p[i]) p[i-1]<p[i] p[i-1] p i-1 i 1 p[i] p i p[i+1]<p[i] p[i+1] p i+1 i 1 p[i] p i ans++ ans System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    }
} public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    }
} Main
import java.util.*;

public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] p = new int[n];
        for (int i=0;i<n;i++)p[i]=sc.nextInt();
        int ans = 0;
        for (int i=1;i<n-1;i++)if (!((p[i-1]>p[i]&&p[i+1]>p[i])||(p[i-1]<p[i]&&p[i+1]<p[i])))ans++;
        System.out.println(ans);
    }
}
