#include <iostream>
#include <vector>

using namespace std;

int main() {
    int n;
    vector<int> v;
    
    cin >> n ;
    
    int tmp = 0;
    for (int i = 0; i < n; i++) {
        cin >> tmp >> std::ws;
        v.push_back(tmp);
    }
    
    int ct = 0;
    for (int i = 0; i < n - 2 ; i++) {
        if (v[i] != v[i + 1] && v[i + 1] != v[i + 2] && v[i + 2] != v[i]) {
            if ((v[i] < v[i + 1] && v[i] > v[i + 2]) ||
                (v[i] > v[i + 1] && v[i] < v[i + 2]) ||
                (v[i + 2] < v[i] && v[i + 2] > v[i + 1]) ||
                (v[i + 2] > v[i] && v[i + 2] < v[i + 1])) {
                    ct++;
            }
        }
    }
    
    cout << ct << endl;
    
    return 0;
}