#include "bits/stdc++.h"

using namespace std;
using ll = long long;
const double pi = acos(-1);
#define FOR(i,a,b) for (ll i=(a),__last_##i=(b);i<__last_##i;i++)
#define RFOR(i,a,b) for (ll i=(b)-1,__last_##i=(a);i>=__last_##i;i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define __GET_MACRO3(_1, _2, _3, NAME, ...) NAME
#define rep(...) __GET_MACRO3(__VA_ARGS__, FOR, REP)(__VA_ARGS__)
#define rrep(...) __GET_MACRO3(__VA_ARGS__, RFOR, RREP)(__VA_ARGS__)
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v) {
	REP(i, v.size()) { if (i)os << " "; os << v[i]; }return os;
}
template<typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v) {
	REP(i, v.size()) { if (i)os << endl; os << v[i]; }return os;
}

int main() {
	ll n;
	ll ans = 0;

	cin >> n;

	vector<ll> v(n);

	REP(i, n) {
		cin >> v[i];
	}

	REP(i, n - 2) {
		if ((v[i] != v[i + 2] && v[i + 1] > v[i] && v[i + 1] > v[i + 2]) || (v[i] != v[i + 2] && v[i + 1] < v[i] && v[i + 1] < v[i + 2])) {
			ans++;
		}
	}

	cout << ans << endl;
}