import java.util.Scanner;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] a = new int[n];
        for(int i=0; i<n; i++) {
            a[i] = in.nextInt();
        }

        int count = 0;
        for(int i=0; i<n-2; i++) {
            if((a[i] < a[i+1] && a[i] > a[i+2]) || (a[i+2] < a[i] && a[i+2] > a[i])) {
                count++;
            }
        }

        System.out.println(count);

        in.close();
    }
}