#include <iostream>

using namespace std;

int main()
{
	int N, a[3], ans = 0;
	cin >> N >> a[0] >> a[1];
	for (int i = 2; i < N; i++) {
		cin >> a[2];
		ans += (a[0] == a[2] || (a[0] < a[1] && a[1] < a[2]) || (a[0] > a[1] && a[1] > a[2])) ? 0 : 1;
		a[0] = a[1]; a[1] = a[2];
	}
	cout << ans << endl;
	return 0;
}