#include <bits/stdc++.h>

using namespace std;

#define REP(i, n) for(int i = 0; i < n; i++)

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int n;
    cin >> n;
    int a[n];
    REP(i, n) cin >> a[i];

    int ans = 0;
    REP(i, n - 2) {
        if (a[i] != a[i + 2] && (a[i + 1] - a[i + 2]) * (a[i + 1] - a[i]) > 0)
            ans++;
    }
    cout << ans << endl;
    return 0;
}