import java.util.Scanner;

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