#include <iostream>	//cin, cout
#include <vector>	//vector
#include <algorithm> //sort,min,max
#include <string>	//string
#include <ios>		//fixed
#include <iomanip>	//setprecision
#include <utility> //swap
#include <cstdlib>	//abs(int)
#include <cmath>	//sqrt
#include <sstream>	//stringstream,getline

using namespace std;

int main() {

	int N;
	cin >> N;

	vector<int> A(N);
	cin >> A[0];
	cin >> A[1];

	int ans = 0;
	for (int i = 2; i < N; i++) {
		cin >> A[i];
		if (A[i] != A[i - 1]) {
			if (A[i - 1] != A[i - 2]) {
				if (A[i] != A[i - 2]) {
					if (max({ A[i - 2], A[i - 1], A[i] }) == A[i - 1]) {
						ans++;
					}
					else if (min({ A[i - 2], A[i - 1], A[i] }) == A[i - 1]) {
						ans++;
					}
				}
			}
		}
	}

	cout << ans << endl;

	return 0;

}