#include<bits/stdc++.h>
using namespace std;
using Int=long long;
using Real=long double;
template<class T,class S>inline bool chmin(T&A,S B){return(A>B?A=B,1:0);}
template<class T,class S>inline bool chmax(T&A,S B){return(A<B?A=B,1:0);}
#define REP(i,n)for(int i=0;i<(n);++i)
#define REP1(i,n)for(int i=1;i<=(n);++i)
#define ALL(x)begin(x),end(x)
#define drop(x){cout<<(x)<<'\n';return;}
int N,a,b,c,ans;
void sol()
{
	cin>>N>>a>>b;
	REP(i,N-2)
	{
		cin>>c;
		if(a!=b&&a!=c&&b!=c&&(max(a,max(b,c))==b||min(a,min(b,c))==b))++ans;
		a=b,b=c;
	}
	cout<<ans<<"\n";
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	cout<<fixed<<setprecision(16);
	int T=1;
	//cin>>T;
	while(T--)sol();
}