#include <iostream>
#include <vector>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n); i++)
#define rep2(i,n) for(int i = 0; i < (n-2); i++)

int main(){
    int cnt = 0;
    int n; cin >> n;
    vector<int> a(n);
    rep(i,n){ cin >> a[i]; }
    rep2(i,n){
        //if(a[i]!=(a[i+1]!=a[i+2])){
        if(a[i]!=a[i+2]){
            if((a[i+1] < a[i+2] && a[i+1] < a[i]) || (a[i+2] < a[i+1] && a[i] < a[i+1])){ cnt++; }
        }
    }
    cout << cnt << endl;
    return 0;
}