import java.util.*;

public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                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();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    }

        System.out.println(count);
  }
} Main public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    }

        System.out.println(count);
  } main {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                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 int[] num = new int[N]; num new int[N] N int count = 0; count 0 for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    } int i=0; int i=0; i 0 i < N i N i++ i++ i {
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    } {
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    } num[i] = sc.nextInt() num[i] num i sc.nextInt() sc.nextInt sc if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        } i > 1 i 1 {
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        } if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            } num[i-2] > num[i-1] && num[i-1] > num[i] num[i-2] > num[i-1] num[i-2] num i-2 i 2 num[i-1] num i-1 i 1 num[i-1] > num[i] num[i-1] num i-1 i 1 num[i] num i {
                count++;
            } count++ count if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            } num[i-2] < num[i-1] && num[i-1] < num[i] num[i-2] < num[i-1] num[i-2] num i-2 i 2 num[i-1] num i-1 i 1 num[i-1] < num[i] num[i-1] num i-1 i 1 num[i] num i {
                count++;
            } 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();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    }

        System.out.println(count);
  }
} public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                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();
    int[] num = new int[N];
    int count = 0;

    for(int i=0; i < N; i++){
        num[i] = sc.nextInt();
        if(i > 1){
            if(num[i-2] > num[i-1] && num[i-1] > num[i]){
                count++;
            }else if(num[i-2] < num[i-1] && num[i-1] < num[i]){
                count++;
            }
        }
    }

        System.out.println(count);
  }
}