#include<iostream>
#include<string>
#include<cstring>
#include<complex>
#include<vector>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<iomanip>
using namespace std;

#define REP(i, j) for (int i = 0; i < (int)j; ++i)
#define FOR(i, j, k) for (int i = (int)j; i < (int)k; ++i)
#define SORT(v) sort(v.begin(), v.end())
#define REVERSE(v) reverse(v.begin(), v.end())
typedef complex<double> P;
typedef vector<int> VI;
typedef long long LL;
typedef pair<int,int> PII;

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n;
	cin >> n;
	VI v;
	REP(i,n) {
		int a;
		cin >> a;
		v.push_back(a);
	}

	int cnt = 0;
	FOR(i,1,n-1) {
		if (v[i] == v[i-1] || v[i] == v[i+1] || v[i-1] == v[i+1]) continue;

		if ((v[i] < v[i-1] && v[i] < v[i+1]) || (v[i] > v[i-1] && v[i] > v[i+1])) cnt++;
	}

	cout << cnt << endl;

	return 0;
}