import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
} Main public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    } main {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[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 int[] array = new int[n]; array new int[n] n for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < n i n i++ i++ i {
            array[i] = sc.nextInt();
        } {
            array[i] = sc.nextInt();
        } array[i] = sc.nextInt() array[i] array i sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        } int i = 1; int i = 1; i 1 i < n - 1 i n - 1 n 1 i++ i++ i {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        } {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        } if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            } array[i-1] < array[i] array[i-1] array i-1 i 1 array[i] array i {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } if (array[i] < array[i+1]) {
                    count++;
                } array[i] < array[i+1] array[i] array i array[i+1] array i+1 i 1 {
                    count++;
                } count++ count {
                if (array[i] > array[i+1]) {
                    count++;
                }
            } if (array[i] > array[i+1]) {
                    count++;
                } array[i] > array[i+1] array[i] array i array[i+1] array i+1 i 1 {
                    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) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int[] array = new int[n];
        for (int i = 0; i < n; i++) {
            array[i] = sc.nextInt();
        }
        
        int count = 0;
        for (int i = 1; i < n - 1; i++) {
            if (array[i-1] < array[i]) {
                if (array[i] < array[i+1]) {
                    count++;
                }
            } else {
                if (array[i] > array[i+1]) {
                    count++;
                }
            }
        }
        System.out.println(count);
    }
}
