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