import java.io.*;
import java.util.*;
import java.util.stream.*;

class Process {
    private int[] A;
    
    Process(int[] A) {
        this.A = A;
    }

    // 3 つの数のうち, num2 が最大かどうか
    private boolean isMiddleMax(int num1, int num2, int num3) {
        return ((num2 > num1) && (num2 > num3));
    }

    // 3 つの数のうち, num2 が最小かどうか
    private boolean isMiddleMin(int num1, int num2, int num3) {
        return ((num2 < num1) && (num2 < num3));
    }
    
    private boolean isKadomatsuSequence(int num1, int num2, int num3) {
        return ((isMiddleMax(num1, num2, num3) || isMiddleMin(num1, num2, num3)) && (num1 != num3));
    }

    int getResult() {
        int count = 0;
        for(int i = 0; i < (A.length - 2); i++) {
            if(isKadomatsuSequence(A[i], A[i + 1], A[i + 2])) {
                count++;
            }
        }

        return count;
    }
}

public class Main {
    public static void main(String[] args) throws IOException {
        var bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        var printWriter    = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));

        // 入力
        int N = Integer.parseInt(bufferedReader.readLine().trim());
        
        int[] A = Stream.of(bufferedReader.readLine().trim().split("[ ]+")).mapToInt(Integer::parseInt).toArray();
        
        // 処理および出力
        printWriter.println((new Process(A)).getResult());

        bufferedReader.close();
        printWriter.close();
    }
}