#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<cmath>
#include<map>
#include<cstdio>
#include<stack>
#include<queue>
#include<fstream>
#include<deque>
#include<iomanip>
#include <numeric>
#include<tuple>
#include <functional>


using namespace std;
#define ok1 printf("ok1\n");
#define ok2 printf("ok2\n");
#define M 1000000000000000000LL
#define rep(i,n) for(int i=0;i<n;++i)
#define REP(i,s,n) for(int i=(s);i<(n);++i)
#define repr(i,n) for(int i=n-1;i>=0;--i)
#define REPR(i,s,n) for(int i=(s);i>=(n);--(i))
#define all(a) (a).begin(),(a).end()
#define reall(a) (a).rbegin(),(a).rend()
#define pb push_back
#define pf push_front
#define MIN(a,b) a=min((a),(b))
#define MAX(a,b) a=max((a),(b))
#define SIZE(v) (int)v.size()
#define DOUBLE fixed << setprecision(10)
#define fi first
#define se second
const double pi = acos(-1.0);
typedef vector<int> vi;
typedef vector<string> vs;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef vector<char> vc;
typedef vector<double> vd;
typedef vector<bool> vb;
typedef deque<ll> dll;
typedef pair<ll, ll> P;
typedef vector<P> vP;
using Edge = pair<int, ll>;
using Graph = vector<vector<Edge> >;
const ll mod = 1e9 + 7;
ll dx[4] = { 1,0,-1,0 };
ll dy[4] = { 0,1,0,-1 };
template <typename T>
bool chmin(T& a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
};
template <typename T>
bool chmax(T& a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
};
void Pvll(vll v) {
	cout << "------------------------------------------------\n";
	rep(i, v.size()) cout << v[i] << " ";
	cout << endl;
	cout << "------------------------------------------------\n";
}
void Pvvll(vvll v) {
	cout << "------------------------------------------------\n";
	rep(i, v.size()) {
		rep(j, v[i].size()) {
			cout << v[i][j] << " ";
		}
		cout << endl;
	}
	cout << "------------------------------------------------\n";
}

void Ps(string s) {
	cout << "------------------------------------------------\n";
	rep(i, s.size()) cout << s[i] << " ";
	cout << endl;
	cout << "------------------------------------------------\n";
}

void Pvs(vs s) {
	cout << "------------------------------------------------\n";
	rep(i, s.size()) {
		rep(j, s[i].size()) {
			cout << s[i][j] << " ";
		}
		cout << endl;
	}
	cout << "------------------------------------------------\n";
}

void Yes(bool x) {
	if (x) cout << "Yes\n";
	else cout << "No\n";
}

void YES(bool x) {
	if (x) cout << "YES\n";
	else cout << "NO\n";
}

void yes(bool x) {
	if (x) cout << "yes\n";
	else cout << "no\n";
}

void Yay(bool x) {
	if (x) cout << "Yay!\n";
	else cout << ":(\n";
}
/*
bool isOK(int mid, double key) {
	double y = (double)mid / (double)n;
	//cout << sqrt(key + y * y) << " ";
	if (sqrt(key + (y * y)) <= 1) return true;
	else return false;
}

int binary_search(double key) {
	int ng = -1; //「index = 0」が条件を満たすこともあるので、初期値は -1
	int ok = n; // 「index = a.size()-1」が条件を満たさないこともあるので、初期値は a.size()
				// ok と ng のどちらが大きいかわからないことを考慮
	while (abs(ok - ng) > 1) {
		int mid = (ok + ng) / 2;
		//cout << mid << " " << key << endl;
		if (!isOK(mid, key)) ok = mid;
		else ng = mid;
	}

	return ok;
}
*/
ll gcd(ll x, ll y)
{
	ll r;
	while ((r = x % y) != 0)
	{
		x = y;
		y = r;
	}
	return y;
}

ll lcm(ll x, ll y)
{
	return (x * y / gcd(x,y));
}



ll  d, n, m, r, l, k, h, w, ans = 0, ret = M;
bool flag = false, flag2 = false, flag3 = false;
//string s,t;

void INIT() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	cout << DOUBLE;
}


int main() {
	INIT();
	/*
	cin >> s;
	map<char, int> m;
	rep(i, 4) m[s[i]]++;
	for (auto p : m) if (p.second != 2) flag = true;
	Yes(!flag);*/
	cin >> n;
	vll s(n);
	rep(i, n) cin >> s[i];
	REP(i, 1, n - 1) {
		if (s[i] > s[i - 1] && s[i] < s[i + 1]) ans++;
		if (s[i] > s[i + 1] && s[i] < s[i - 1]) ans++;
	}
	cout << ans << endl;
	return 0;
}


Measured memory_bytes: 256