#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < n; ++i)
using ll = long long;
using namespace std;
const int INF = 1e9;

int main() {
    int n;
    cin >> n;
    vector<int> a(n);
    rep(i, n) cin >> a[i];

    int ans = 0;
    rep(i, n - 2) {
        if (a[i] != a[i + 1] && a[i + 1] != a[i + 2] && a[i] != a[i + 2]) {
            int tmp = (a[i + 1] - a[i]) * (a[i + 1] - a[i + 2]);
            if (tmp > 0)
                ++ans;
        }
    }

    cout << ans << endl;

    return 0;
}