#include <iostream>
#include <vector>
#include <algorithm>

#define ALL(x) (x).begin(),(x).end()

using namespace std;

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