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];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} 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];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n=sc.nextInt();
		int[] nums = new int[n];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int n=sc.nextInt();
		int[] nums = new int[n];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	} 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 int ans=0; ans 0 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 for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		} int i=0; int i=0; i 0 i<n-2 i n-2 n 2 i++ i++ i {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		} {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		} if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			} nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i] nums[i]<nums[i+1] && nums[i+1]<nums[i+2] nums[i]<nums[i+1] nums[i] nums i nums[i+1] nums i+1 i 1 nums[i+1]<nums[i+2] nums[i+1] nums i+1 i 1 nums[i+2] nums i+2 i 2 nums[i+2]<nums[i+1] && nums[i+1]<nums[i] nums[i+2]<nums[i+1] nums[i+2] nums i+2 i 2 nums[i+1] nums i+1 i 1 nums[i+1]<nums[i] nums[i+1] nums i+1 i 1 nums[i] nums i {
				ans++;
			} ans++ ans System.out.println(ans) System.out.println System.out System System.out ans 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];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

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

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	}
} 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];
		int ans=0;

		for(int i=0;i<n;i++) {
			nums[i]=sc.nextInt();
		}

		for(int i=0;i<n-2;i++) {
			if(nums[i]<nums[i+1] && nums[i+1]<nums[i+2] || nums[i+2]<nums[i+1] && nums[i+1]<nums[i]) {
				ans++;
			}
		}
		System.out.println(ans);
	}
}