#include <iostream>
using namespace std;

int main()
{
	const int MAX = 100;
	int n, A[MAX];
	cin >> n;
	int ans = 0;
	for (int i = 0; i < n; i++)
	{
		cin >> A[i];
		if (i >= 2)
		{
			int a = A[i - 2], b = A[i - 1], c = A[i];
			if (b > a && b > c && a != c)
			{
				ans++;
			}
			if (b < a && b < c && a != c)
			{
				ans++;
			}
		}
	}
	cout << ans << endl;
}