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