#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <iomanip>
#include <cmath>
#include <functional>
using namespace std;
#define diff(x,y) ((x > y) ? (x - y) : (y - x))
#define us(x) (x > 0) ? x : x * -1
#define lli long long int

int main() {
	int n;
	int ans = 0;
	vector <int> x;
	cin >> n;
	vector <int> a(n, 0);
	for (int i = 0; i < n; i++)
		cin >> a[i];
	for (int i = 0; i < n - 2; i++) {
		x.push_back(a[i]);	x.push_back(a[i + 1]);	x.push_back(a[i + 2]);
		sort(x.begin(), x.end());
		if ((x[1] == a[i] || x[1] == a[i + 2]) && (x[0] != x[1] && x[1] != x[2]))
			ans++;
		x.clear();
	}
	cout << ans << endl;

	return 0;
}