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