#include<iostream>
#include<vector>
using namespace std;
int main(){
    int n;
    cin>>n;
    vector<int> a(n);
    int c=0;
    for(int i=0;i<n-2;i++){
        if((a[i]<a[i+1]&&a[i+1]>a[i+2]&&a[i]!=a[i+1])||(a[i]>a[i+1]&&a[i+1]<a[i+2]&&a[i]!=a[i+2])){
            c++;
        }
    }
    cout<<c<<endl;
}