import java.util.*;
import java.io.*;

public class Main
{
	public static void main(String [] Args)
	{
		Scanner in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	}
}
	 import java.util.*; util.* java import java.io.*; io.* java public class Main
{
	public static void main(String [] Args)
	{
		Scanner in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main public static void main(String [] Args)
	{
		Scanner in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	} main {
		Scanner in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	} Scanner in=new Scanner(System.in); in new Scanner(System.in) int n=in.nextInt(); n in.nextInt() in.nextInt in int [] p=new int [n]; p new int [n] n for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			p[i]=in.nextInt();
		} {
			p[i]=in.nextInt();
		} p[i]=in.nextInt() p[i] p i in.nextInt() in.nextInt in int ans=0; ans 0 for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				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]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		} {
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		} if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			} (p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]) (p[i]>p[i-1]&&p[i]<p[i+1]) p[i]>p[i-1] p[i] p i p[i-1] p i-1 i 1 p[i]<p[i+1] p[i] p i p[i+1] p i+1 i 1 (p[i]>p[i+1]&&p[i]<p[i-1]) p[i]>p[i+1] p[i] p i p[i+1] p i+1 i 1 p[i]<p[i-1] p[i] p i p[i-1] p i-1 i 1 {
				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 in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	}
} public class Main
{
	public static void main(String [] Args)
	{
		Scanner in=new Scanner(System.in);		
		int n=in.nextInt();
		int [] p=new int [n];
		for(int i=0;i<n;i++)
		{
			p[i]=in.nextInt();
		}
		int ans=0;
		for(int i=1;i<n-1;i++)
		{
			if((p[i]>p[i-1]&&p[i]<p[i+1])||(p[i]>p[i+1]&&p[i]<p[i-1]))
			{
				ans++;
			}
		}
		System.out.println(ans);
	}
} Main
import java.util.*;
import java.io.*;

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