#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <iomanip>
using namespace std;
using ll = long long;

int main() {
    int N;
    cin >> N;
    vector<int> a(N);
    for(int i=0; i<N; ++i) {
        cin >> a[i];
    }
    int res = 0;
    for(int i=0; i<N-2; ++i) {
        if(a[i] != a[i+2] && (a[i] < a[i+1] && a[i+2] < a[i+1] || a[i] > a[i+1] && a[i+2] > a[i+1])) {
            res++;
        }
    }
    cout << res << endl;
}