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