#include <iostream>
#include <vector>

void solve() {
    int n;
    std::cin >> n;

    std::vector<int> xs(n);
    for (auto& x : xs) std::cin >> x;

    int ans = 0;
    for (int i = 0; i + 2 < n; ++i) {
        int j = i + 1, k = i + 2;
        if (((xs[i] < xs[j] && xs[j] > xs[k]) ||
             (xs[i] > xs[j] && xs[j] < xs[k])) &&
            xs[i] != xs[k]) ++ans;
    }

    std::cout << ans << std::endl;
}

int main() {
    std::cin.tie(nullptr);
    std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}