#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;

bool solve(int a, int b, int c) {

    return (a < c and c < b) or
           (b < a and a < c) or
           (b < c and c < a) or
           (c < a and a < b);
}


int main() {


    int N;
    cin>>N;

    vector<int> as;
    for (int i = 0; i < N; ++i) {
        int A;
        cin>>A;
        as.push_back(A);
    }

    int cnt = 0;
    for (int i = 2; i < N; ++i) {
        if (solve(as[i - 2], as[i - 1], as[i])) {
            cnt += 1;
        }
    }

    cout << cnt << endl;


    return 0;

}