#include <iostream>
#include <vector>
using namespace std;
int main(int argc, char* argv[])
{
	int N;
	cin>>N;
	int i;
	vector<int> A(N);
	for (i=0;i<N;i++){
		cin>>A[i];
	}
	int ans=0;
	for (i=0;i<N-2;i++){
		if (A[i]<=A[i+1] && A[i+1]<=A[i+2] || A[i]>=A[i+1]&& A[i+1]>=A[i+2] || A[i]==A[i+1] || A[i+1]==A[i+2] || A[i]==A[i+2]){
		}else{
			//cout<<A[i]<<" "<<A[i+1]<<" "<<A[i+2]<<endl;
			ans++;
		}
	}
	cout<<ans<<endl;
	return 0;
}