#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;

int main() {
    int n; cin >> n;
    vector<int> A(n);
    for (int &a: A) cin >> a;
    int cnt = 0;
    for (int i = 0; i + 2 < n; i++) {
        int a = A[i], b = A[i + 1], c = A[i + 2];
        cnt += (a != c && (b - a) * (b - c) > 0);
    }
    cout << cnt << endl;
    return 0;
}