#include<stdio.h>

int main(void){
	int i,j,k,n,a[128]={0},r=0;
	scanf("%d",&n);
	for(i = 1;i <= n;i++){scanf("%d",&a[i]);}
	for(i = 1;i <= n-2;i++){
		if(((a[i] < a[i+1] && a[i+1] > a[i+2]) || (a[i] > a[i+1] && a[i+1] < a[i+2])) && a[i] != a[i+2]){r++;}
	}
	printf("%d\n",r);
	return 0;
}