#define _USE_MATH_DEFINES

#include <iostream>		//cin, cout
#include <vector>		//vector
#include <algorithm>	//sort,min,max,count
#include <string>		//string,getline, to_string
#include <cstdlib>		//abs(int)
#include <utility>		//swap, pair
#include <deque>		//deque
#include <climits>		//INT_MAX
#include <bitset>		//bitset
#include <cmath>		//sqrt, ceil. M_PI, pow, sin
#include <ios>			//fixed
#include <iomanip>		//setprecision
#include <sstream>		//stringstream
#include <numeric>		//gcd, assumlate
#include <random>		//randam_device
#include <limits>		//numeric_limits

using namespace std;
constexpr long long int D_MOD = 1000000007;

inline bool JudgeKadomatsu(int a, int b, int c) {
	//門松列か判定する

	if (a == b || b == c || c == a) {
		return false;
	}
	if (b == max({ a, b, c }) || b == min({ a, b, c })) {
		return true;
	}
	else {
		return false;
	}
}

int main() {

	int N;
	cin >> N;

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

	int ans = 0;

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

	cout << ans << endl;

	return 0;

}