#include <stdio.h>
int main(){
int a,b,c,n,s=0;
scanf("%d%d%d",&n,&a,&b);n-=2;
while(n--){
	scanf("%d",&c);
	if(a-c&&(a<b&&c<b||b<a&&b<c))s++;
	a=b;b=c;
}
printf("%d",s);
return 0;
}