#include<bits/stdc++.h>
using namespace std;
int n,ans;
int a[22];
int main() {
	scanf("%d",&n);
	for(int i=1;i<=n;i++)scanf("%d",&a[i]);
	for(int i=2; i<n; i++) {
		if((a[i]>a[i-1]&&a[i]<a[i+1])||(a[i]<a[i-1]&&a[i]>a[i+1]))ans++;
	}
	printf("%d",ans);
}
