#include <bits/stdc++.h>
using namespace std;
 
int main(){
    int N;
    cin>>N;
    vector<int> take(N);
    int count=0;
    for(int i=0;i<N;i++){
        cin>>take[i];
    }
    for(int i=0;i<N-2;i++){
        if(take[i]!=take[i+1] && take[i+1]!=take[i+2] && take[i]!=take[i+2]){
            if(take[i]>take[i+1] && take[i+1]<take[i+2]){
                count++;
                
            }
            else if(take[i]<take[i+1] && take[i+1]>take[i+2]){
                count++;
            }
        }
    }
    cout<<count<<endl;
}