#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>

using namespace std;

int main() {
	int n;

	cin >> n;
	vector<int> a(n, 0);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}

	int ans = 0;
	int left, center, right;
	for (int i = 2; i < n; i++) {
		left = a[i - 2];
		center = a[i - 1];
		right = a[i];

		if (left == center || center == right || right == left) {
			continue;
		}

		if (left < center && center < right) {
			continue;
		}
		if (left > center && center > right) {
			continue;
		}
		ans++;
	}

	cout << ans << endl;
	return 0;
}