#include <iostream>
#include <vector>


using namespace std ;
int main(){
    int n ,a,sum = 0;



    vector <int> seq ;
    cin >> n;
    for(int i = 0 ;i< n ;i++){
        cin >> a;
        seq.push_back(a);
    }

    for (int i = 0; i<n-2 ;i++){//原本是寫(int i = 0 ; i< seq.size()-2;i++)
        if((seq[i+1]<seq[i]&&seq[i+1]<seq[i+2])||(seq[i+1]>seq[i]&&seq[i+1]>seq[i+2])) sum ++ ;

    }

    cout << sum ;
    return 0 ;
}