#define _USE_MATH_DEFINES	//M_PI
#include <iostream>			//std::cout, std::cin
#include <string>			//std::string
#include <vector>			//std::vector
#include <valarray>			//std::valarray	数値のみの一次配列
#include <algorithm>		//std::sort
#include <time.h>			//localtime_s
#include <cstdlib>			//abs
#include <cmath>			//abs, std::pow, sqrt, sin, cos,
#include <fstream>			//std::ifstream
#include <iomanip>			//std::setprecision
#include <random>			//std::random(C++11)
#include <numeric>			//std::accumulate

int main(void) {

	//test用
	//std::ifstream in("test.txt");
	//std::cin.rdbuf(in.rdbuf());

	int N;
	std::cin >> N;
	std::vector<int> A(N);
	for (int i = 0; i < N; i++) {
		std::cin >> A[i];
	}

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

	std::cout << ans << std::endl;

}