#include <iostream>
using namespace std;

int main() {
	int a,b,c,i,n,co;
	cin >> n >> a >> b >> c;
	co = 0;
	if (a < b && b > c && a != c) {
		co++;
	}
	else if (a > b && b < c && a != c) {
		co++;
	}
	for ( i = 1; i <= n - 3; i++) {
		a = b;
		b = c;
		cin >> c;
		if (a < b && b > c && a != c) {
		co++;
	}
	else if (a > b && b < c && a != c) {
		co++;
	}
	}
	cout << co << endl;
	return 0;
}