#include <iostream>
using namespace std;

int main(){
	int n; cin >> n;
	int a[n];
	for(int i=0;i<n;i++){cin >> a[i];}
	int cnt=0;
	for(int i=1;i<n-1;i++){
		if(a[i-1]==a[i+1])continue;
		if(a[i]>a[i-1]){ if(a[i]>a[i+1]) cnt++;}
		else if(a[i]<a[i-1]){ if(a[i]<a[i+1]) cnt++;}
	}
	cout << cnt << endl;
	return 0;
}