#include<iostream>
#include<cmath>
using namespace std;
int main()
{
	int c=0,n,a[114];
	cin>>n;
	for(int i=0;i<n;i++)cin>>a[i];
	for(int i=1;i<n-1;i++)if((a[i-1]<a[i]&&a[i]<a[i+1])||(a[i-1]>a[i]&&a[i]>a[i+1]))c++;
	cout<<c<<endl;
	return 0;
}