import java.util.*;

public class Main
{
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		}

		System.out.println(ans);
	}

} 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 p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		}

		System.out.println(ans);
	}

} Main public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		}

		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				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 p[]; p p = new int[20] p new int[20] 20 for(int i = 0 ; i < n ; i++)
		{
			p[i] = sc.nextInt();
		} int i = 0 ; int 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 int ans = 0; ans 0 for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		} int i = 1 ; int i = 1 ; i 1 i < n-1 i n-1 n 1 i++ i++ i {
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		} {
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		} if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			} ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) ((p[i] - p[i-1]) > 0) (p[i] - p[i-1]) p[i] p i p[i-1] p i-1 i 1 0 ((p[i] - p[i+1]) > 0) (p[i] - p[i+1]) p[i] p i p[i+1] p i+1 i 1 0 {
				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 p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				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 p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		}

		System.out.println(ans);
	}

} Main
import java.util.*;

public class Main
{
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();
		int p[];
		//N <= 20
		p = new int[20];

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

		int ans = 0;
		for(int i = 1 ; i < n-1 ; i++)
		{
			if( ((p[i] - p[i-1]) > 0) ^ ((p[i] - p[i+1]) > 0) )
			{
				ans++;
			}
		}

		System.out.println(ans);
	}

}