#include<stdio.h>
#include<algorithm>
using namespace std;
int b[110];
int main(){
	int a;scanf("%d",&a);
	for(int i=0;i<a;i++)scanf("%d",b+i);
	int ret=0;
	for(int i=0;i<a-2;i++){
		if(b[i]==b[i+1]||b[i]==b[i+2]||b[i+1]==b[i+2])continue;
		if((b[i+1]-b[i])*(b[i+2]-b[i])<0)ret++;
		else if((b[i+1]-b[i+2])*(b[i]-b[i+2])<0)ret++;
	}
	printf("%d\n",ret);
}