inline bool kadomatu(int a,int b,int c){
    return a!=b&&b!=c&&c!=a&&((a<b && c<b)||(a>b && c>b));
}

{
  int i,n,a[1000],c=0;
  rd(n);rd(a(n));
  rep(i,0,n-3){
    c+=kadomatu(a[i],a[i+1],a[i+2]);
  }
  wt(c);
}