#define _USE_MATH_DEFINES
#include<stdio.h>
#include<string>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include<iomanip>

//#include<bits/stdc++.h>



using namespace std;




int main() {

	int n;
	cin>>n;
	vector<int>m(n);
	for(int i=0;i<n;i++)
		cin>>m[i];
	int ans=0;
	for(int i=0;i<n-2;i++){
		int a=m[i],b=m[i+1],c=m[i+2];
		if(a<=b&&b<=c)continue;
		if(a>=b&&b>=c)continue;
		if(a!=c)ans++;
	}
	cout<<ans<<endl;
	return 0;
}