import java.math.*;
import java.util.*;

public class Main {
	static boolean check(int a,int b,int c){
		if((a>b && b>c) || (a<b && b< c) || a==b || b==c || c==a) return false;
		return true;
	}
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] ma = new int[n];
        for(int i=0;i<n;i++){
        	ma[i]=sc.nextInt();
        }
        int ans=0;
        for(int i=0;i<=n-3;i++){
        	if(check(ma[i], ma[i+1], ma[i+2])) ans++;
        }
        System.out.println(ans);
    }
}