#include<iostream>
#include<stdio.h>
#include<vector>
#include<algorithm>

using namespace std;

int main(){

    int n;
    cin>> n;

    vector<int> arr(n);
    for(int i=0; i<n; i++){
        cin>> arr[i];
    }

    int cnt = 0;
    for(int i=2; i<n; i++){
        if(arr[i-2]==arr[i-1] || arr[i-1]==arr[i] || arr[i-2]==arr[i]){
            continue;
        }else if(arr[i-1]==max(arr[i-2], max(arr[i-1], arr[i])) || arr[i-1]==min(arr[i-2], min(arr[i-1], arr[i]))){
            cnt++;
        }
    }

    cout<< cnt<< endl;
    return 0;
}