#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

int findmedian(int left,int mid,int right);

int main(void)
{
    int bamboo;
    int lenlist[120]={0};
    scanf("%d",&bamboo);
    for(int i=0;i<bamboo;i++)
    {
        scanf("%d",&lenlist[i]);
    }
    int cnt=0;
    for(int i=0;i<bamboo-2;i++)
    {
        int left=lenlist[i];
        int mid=lenlist[i+1];
        int right=lenlist[i+2];
        if(left==mid||left==right||mid==right)
        {
            continue;
        }
        int median=findmedian(left,mid,right);
        if(left==median||right==median)
        {
            cnt++;
        }
    }
    printf("%d",cnt);
}

int findmedian(int a,int b,int c)
{
    int low=a;
    int hi=b;
    if(a>b)
    {
        low=b;
        hi=a;
    }
    if(c>=hi)
    {
        return hi;
    }
    if(c<=low)
    {
        return low;
    }
    return c;

}