#include "bits/stdc++.h"
using namespace std;
using Graph = vector<vector<int>>;
typedef long long ll;
typedef pair<ll, ll> P;
#define rep(i,n) for (ll i = 0; i < (n); ++i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
const long long mod = 1e9 + 7;

ll gcd(ll a, ll b) {
	return b ? gcd(b, a % b) : a;
}

ll lcm(ll a, ll b) {
	return a / gcd(a, b) * b;
}

int main() {
	ll n;
	cin >> n;

	vector<ll> v(n);
	
	rep(i, n) {
		cin >> v[i];
	}

	ll ans = 0;

	rep(i, n - 2) {
		ll a = v[i];
		ll b = v[i + 1];
		ll c = v[i + 2];
		if (a != b && a != c && b != c) {
			ll MAX = max({a, b, c});
			ll MIN = min({a, b, c});
			if (MAX == b || MIN == b) {
				ans++;
			}
		}
	}

	cout << ans << endl;
}