import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.stream.Stream;

public class No00000116_Main {

	public static void main(String[] args) throws IOException {

		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		int n = Integer.parseInt(br.readLine());
		int[] arr = Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray();

		int cnt = 0;
		for(int i = 1; i < n - 1; i++) {
			cnt += check(arr[i - 1], arr[i], arr[i + 1]);
		}

		System.out.println(cnt);

	}

	private static int check(int num1, int num2, int num3) {
		if(num1 == num2 || num2 == num3 || num3 == num1) {
			return 0;
		}
		if(Math.max(Math.max(num1, num2), num3) == num2
				|| Math.min(Math.min(num1, num2), num3) == num2) {
			return 1;
		}
		return 0;
	}

}