import java.util.Scanner;
public class Main {

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

} import java.util.Scanner; Scanner java.util public class Main {

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

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

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

} public class Main {

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

} Main
import java.util.Scanner;
public class Main {

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

}
