#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;

const int N = 1010;
int n;
int a[N];

int main()
{
    cin>>n;
    for(int i = 0; i<n; i++) cin>>a[i];
    int ans = 0;
    for(int i = 1; i+1<n; i++)
    {
        if((a[i]<a[i-1]&&a[i]<a[i+1]) || (a[i]>a[i-1]&&a[i]>a[i+1]))
            ans++;
    }
    cout<<ans;
    return 0;
}