#include <iostream>
using namespace std;
int main() {
	int N;
	int A[100] = { 0 };
	int order[3] = { 0 };
	int ans = 0;
	cin >> N;
	for (int i = 0; i < N;i++) {
		cin >> A[i];
	}
	int key = 2;

	do {
		if (A[key - 2] >= A[key -1]) {
			if (A[key - 2] >= A[key]) {
				if (A[key - 1] >= A[key]) {
					order[0] = 1; order[1] = 2; order[2] = 3;
				}
				else {
					order[0] = 1; order[1] = 3; order[2] = 2;
				}
			}
			else {
				order[0] = 2; order[1] = 3; order[2] = 3;
			}
		}
		else {
			if (A[key - 2] >= A[key]) {
				order[0] = 2; order[1] = 1; order[2] = 3;
			}
			else {
				if (A[key - 1] >= A[key]) {
					order[0] = 3; order[1] = 1; order[2] = 2;
				}
				else {
					order[0] = 3; order[1] = 2; order[2] = 1;
				}
			}
		}
		if ((order[0] == 2) && A[key - 2] != A[key - 1]) {
			if (A[key - 2] != A[key]) {
				ans++;
			}
		}
		else if ((order[2] == 2) && A[key] != A[key - 1]) {
			if (A[key - 2] != A[key]) {
				ans++;
			}
		}
		key++;
	} while (key < N);
	cout << ans << endl;
	return 0;
}