#include<iostream>
#include<string>
#include<cmath>
#include<cstdio>

using namespace std;

bool judge(int A1,int A2,int A3){
	return ((A1>A2&&A2<A3)||(A1<A2&&A2>A3))&&A1!=A2&&A2!=A3&&A1!=A3;
}

int main(){

	int N;
	int A1,A2,A3,Ain;
	int ans=0;

	cin>>N;
	cin>>A3>>A2>>A1;
	if(judge(A1,A2,A3)) ans++;

	for(int i=4;i<=N;i++){
		cin>>Ain;
		A3=A2;
		A2=A1;
		A1=Ain;
		if(judge(A1,A2,A3)) ans++;
	}

	cout<<ans<<endl;
}