import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        int kadomatu = 0;
        int N = Integer.parseInt(reader.readLine());
        String[] inputs = reader.readLine().split(" ");
        for (int i = 0; i < N - 2; i++) {
            int a = Integer.parseInt(inputs[i]);
            int b = Integer.parseInt(inputs[i + 1]);
            int c = Integer.parseInt(inputs[i + 2]);
            if (isKadomatu(a, b, c)) {
                kadomatu += 1;
            }
        }
        System.out.println(kadomatu);
    }

    private static boolean isKadomatu(int a, int b, int c) {

        if (a != b && a != c && b != c) {

            int middle;
            if (a <= b && b <= c) {
                middle = b;
            } else if (b <= a && a <= c) {
                middle = a;
            } else {
                middle = c;
            }

            if (middle == a || middle == c) {
                return true;

            } else {
                return false;
            }

        } else {
            return false;
        }
    }
}