#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;

int main() {
	int n, a[100];
	cin >> n;

	int ans = 0;
	cin >> a[0] >> a[1];
	for (int i = 2; i < n; i++)
	{
		cin >> a[i];
		if ((a[i-2] != a[i-1]) && (a[i - 1] != a[i]) && (a[i-2] != a[i]))
		{
			bool temp1 = a[i - 2] > a[i - 1];
			bool temp2 = a[i - 1] < a[i];
			ans += (temp1 == temp2) ? 1 : 0;
		}
	}
	cout << ans << endl;

	return 0;
}