import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	}
} import java.util.*; util.* java public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	}
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	} main {
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc int p[] = new int[n]; p new int[n] n int i = 0; i 0 int count = 0; count 0 for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      } i = 0; i = 0 i 0 i < n i n i++ i++ i {
        p[i] = sc.nextInt();
      } {
        p[i] = sc.nextInt();
      } p[i] = sc.nextInt() p[i] p i sc.nextInt() sc.nextInt sc for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      } i = 1; i = 1 i 1 i < n-1 i n-1 n 1 i++ i++ i {
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      } {
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      } if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        } p[i-1] < p[i] && p[i + 1] > p[i] p[i-1] < p[i] p[i-1] p i-1 i 1 p[i] p i p[i + 1] > p[i] p[i + 1] p i + 1 i 1 p[i] p i {
          count += 1;
        } count += 1 count 1 if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        } p[i-1] > p[i] && p[i + 1] < p[i] p[i-1] > p[i] p[i-1] p i-1 i 1 p[i] p i p[i + 1] < p[i] p[i + 1] p i + 1 i 1 p[i] p i {
          count += 1;
        } count += 1 count 1 System.out.println(count) System.out.println System.out System System.out count String[] args args public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	}
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	}
} Main
import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力
		int n = sc.nextInt();
        int p[] = new int[n];
        int i = 0;
        int count = 0;
		// 出力
      for(i = 0; i < n; i++){
        p[i] = sc.nextInt();
      }
	  for(i = 1; i < n-1; i++){
        if(p[i-1] < p[i] && p[i + 1] > p[i]){
          count += 1;
        }else if(p[i-1] > p[i] && p[i + 1] < p[i]){
          count += 1;
        }
      }
      System.out.println(count);
	}
}