import java.util.Scanner;

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

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