#include<iostream>
using namespace std;
int main(void){
	int v[97] = { 0 }, n;
	cin >> n;
	for (int i = 0; i < n; i++){ cin >> v[i]; }
	int ans = 0;
	for (int i = 0; i <= n - 3; i++){
		int a = v[i], b = v[i + 1], c = v[i + 2];
		ans += (a < b && c < a);
		ans += (a < c && b < a);
		ans += (c < b && a < c);
		ans += (c < a && b < c);
	}
	cout << ans << endl;
	return 0;
}