import 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];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            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);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        }
         System.out.println(count);
    }
} Main public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        }
         System.out.println(count);
    } main {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            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[] a = new int[n]; a new int[n] n int mid = 0; mid 0 int count = 0; count 0 for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < a.length i a.length a a.length ++i ++i i {
          a[i] = sc.nextInt();
        } {
          a[i] = sc.nextInt();
        } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        } int i = 0; int i = 0; i 0 i <(n-2) i (n-2) n 2 i++ i++ i {
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        } {
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        } mid = a[(i+1)] mid a[(i+1)] a (i+1) i 1 if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            } mid > a[i] && mid < a[(i+2)] mid > a[i] mid a[i] a i mid < a[(i+2)] mid a[(i+2)] a (i+2) i 2 {
            count += 1; 
            } count += 1 count 1 if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            } mid < a[i] && mid > a[(i+2)] mid < a[i] mid a[i] a i mid > a[(i+2)] mid a[(i+2)] a (i+2) i 2 {
            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);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        }
         System.out.println(count);
    }
} public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            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);
		int n = sc.nextInt();    	
      	int[] a = new int[n];
      	int mid = 0;
      	int count = 0;
        for(int i = 0; i < a.length; ++i){
          a[i] = sc.nextInt();
        } 
        for(int i = 0; i <(n-2); i++){
            mid = a[(i+1)];
            if(mid > a[i] && mid < a[(i+2)]){
            count += 1; 
            }if(mid < a[i] && mid > a[(i+2)]){
            count += 1; 
            }
        }
         System.out.println(count);
    }
}
