#include <bits/stdc++.h>

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 = 0; i + 2 < n; ++i) {
        if (a[i] != a[i + 2] and
            (a[i + 1] - a[i + 2]) * (a[i + 1] - a[i]) > 0)
            ans++;
    }

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