【思路】
若斜率之积为负数则该点为转折点

import java.io.*;
public class Main{
    static int N = 1010;
    static int a[] = new int[N];
    public static void main(String args[])throws Exception{
        BufferedReader bf =  new BufferedReader(new InputStreamReader(System.in));
        int  n = Integer.parseInt(bf.readLine());
        String [] s = bf.readLine().split(" ");
        for(int i = 0; i < n ; i ++) a[i] = Integer.parseInt(s[i]);
        int ans = 0;
        for(int i =  1; i < n - 1; i ++)
            //若斜率之积为负数则该点为转折点
            if( (a[i] - a[i - 1]) * (a[ i + 1] - a[i]) < 0) ans++;
        System.out.println(ans);





    }
}