#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream>
#include <random>
#include<map>
#include <fstream>
#include <iomanip>
#include <time.h>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <assert.h>
using namespace std;
#define eps 0.000000001
#define LONG_INF 10000000000000000
#define GOLD 1.61803398874989484820458
#define MAX_MOD 1000000007
#define MOD 998244353
#define seg_size 65536*2
#define REP(i,n) for(long long i = 0;i < n;++i)

int main() {
	int n;
	cin >> n;
	vector<int> a;
	REP(i, n) {
		int b;
		cin >> b;
		a.push_back(b);
	}
	int ans = 0;
	for (int i = 1; i < n - 1; ++i) {
		if (a[i - 1] == a[i + 1]) continue;
		if (a[i] > a[i - 1] && a[i] > a[i + 1]) ans++;
		if (a[i] < a[i - 1] && a[i] < a[i + 1]) ans++;
	}
	cout << ans << endl;
	return 0;
}