#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

int main() {
    int n; cin >> n;
    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] && (a[i+1] < a[i] && a[i+1] < a[i+2] || a[i+1] > a[i] && a[i+1] > a[i+2]))
            ans++;
    cout << ans << endl;
    return 0;
}