#include <iostream>

using namespace std;

const int N = 1010;

int q[N];

int main() {
    int n;
    cin >> n;

    for (int i = 0; i < n; ++i) {
        cin >> q[i];
    }

    int res = 0;
    for (int i = 1; i < n - 1; ++i) {
        int a = q[i], b = q[i-1], c = q[i+1];
        if ((a > b && a > c) || (a < b && a < c)) {
            ++res;
        }
    }

    cout << res << endl;

    return 0;
}