import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int[] p = new int[N];
        
        for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

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

        int count = 0;
        for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        }
        
        System.out.println(count);
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int[] p = new int[N];
        
        for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int count = 0;
        for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        }
        
        System.out.println(count);
    } main {
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int[] p = new int[N];
        
        for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int count = 0;
        for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        }
        
        System.out.println(count);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = Integer.parseInt(sc.next()); N Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int[] p = new int[N]; p new int[N] N for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        } int i = 0; int i = 0; i 0 i < N i N i++ i++ i {
            p[i] = Integer.parseInt(sc.next());
        } {
            p[i] = Integer.parseInt(sc.next());
        } p[i] = Integer.parseInt(sc.next()) p[i] p i Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int count = 0; count 0 for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        } 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] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        } {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        } if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            } p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1] p[i-1] < p[i] && p[i] < p[i+1] p[i-1] < p[i] p[i-1] p i-1 i 1 p[i] p i p[i] < p[i+1] p[i] p i p[i+1] p i+1 i 1 p[i+1] < p[i] && p[i] < p[i-1] p[i+1] < p[i] p[i+1] p i+1 i 1 p[i] p i p[i] < p[i-1] p[i] p i p[i-1] p i-1 i 1 {
                count++;
            } count++ count System.out.println(count) System.out.println System.out System System.out count String[] args args
import java.util.*;

class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int N = Integer.parseInt(sc.next());
        int[] p = new int[N];
        
        for(int i = 0; i < N; i++) {
            p[i] = Integer.parseInt(sc.next());
        }

        int count = 0;
        for(int i = 1; i < N-1; i++) {
            if(p[i-1] < p[i] && p[i] < p[i+1] || p[i+1] < p[i] && p[i] < p[i-1]) {
                count++;
            }
        }
        
        System.out.println(count);
    }
}