import java.util.*;

class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] p = new int[n];
		for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		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.*; util.* java class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] p = new int[n];
		for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		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 sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] p = new int[n];
		for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		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 sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] p = new int[n];
		for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		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 sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int[] p = new int[n]; p new int[n] n for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		} int i=0; int i=0; i 0 i<n i n i++ i++ i {
			p[i] = Integer.parseInt(sc.next());
		} {
			p[i] = Integer.parseInt(sc.next());
		} p[i] = Integer.parseInt(sc.next()) p[i] p i Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc 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
import java.util.*;

class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = Integer.parseInt(sc.next());
		int[] p = new int[n];
		for (int i=0;i<n;i++) {
			p[i] = Integer.parseInt(sc.next());
		}
		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);
	}
}