import java.util.*;

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