#include <iostream>
#include <string>
using namespace std;

int main() {
	int n, a, b, c;
	cin >> n >> b >> c;
	int ans = 0;
	for (int i = 0; i < n - 2; ++i) {
		a = b;
		b = c;
		cin >> c;
		if (a != b && b != c && c != a && !(a > b && b > c || a < b && b < c))
			ans++;
	}
	cout << ans << endl;
}