#include <bits/stdc++.h>
using namespace std;

int main() {
    int n;
    cin>>n;
    int a[n];
    for(int i=0;i<n;i++) cin>>a[i];

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