#include<bits/stdc++.h>
using namespace std;
int main()
{
int n;
  cin>>n;
  int a[n];int c=0;
  for(int i=0;i<n;i++)
  {cin>>a[i];}
 for(int i=1;i<n-1;i++)
 {if((a[i]<a[i-1])&&(a[i]>a[i+1]))
 c++;
 else if((a[i]>a[i-1])&&(a[i]<a[i+1]))
   c++;}
  cout<<c;
}