#include <iostream>
#include <vector>
using namespace std;

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

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

    int ans = 0;
    for (int i = 1; i < n - 1; i++) {
        if (a[i - 1] == a[i + 1]) continue;
        if (a[i] < a[i - 1] && a[i] < a[i + 1]) {
            ans++;
        } else if (a[i] > a[i - 1] && a[i] > a[i + 1]) {
            ans++;
        }
    }
    cout << ans << endl;

    return 0;
}