#include<iostream>
#include <cstdlib>
#include<list>
#include<string>
#include<stack>
using namespace std;
int main() {
	int n, count=0,a[1000]; cin >> n; 
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	for (int i = 0; i < n-2; i++)
	{
		int aa, bb, cc; aa = a[i], bb = a[i + 1], cc = a[i + 2];
		if (aa == bb || aa == cc || bb == cc) {

		}
		else {
			if (bb > aa&&cc > bb) {

			}
			else if(bb<aa&&cc<bb){}
			else {
				count++;
			}
		}
	}
	cout << count;
	return 0;
}