#include<bits/stdc++.h>
using namespace std;

int main(){
    int N,a,b,c,ans=0;

    cin >> N;
    cin >> a >> b;
    for(int i=2;i<N;i++){
        cin >> c;
        if(a!=c && ((a<b&&b>c)||(a>b&&b<c))) ans++;
        a=b;b=c;
    }
    cout << ans << endl;

    return 0;
}