#include <bits/stdc++.h>
using namespace std;
int mid (int a, int b, int c) { return (a + b + c - min({a, b, c}) - max({a, b, c})); }

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+1 < n; i++) {
        if (a[i-1] == a[i] or a[i] == a[i+1] or a[i+1] == a[i-1]) continue;
        ans += (a[i] != mid(a[i-1], a[i], a[i+1]));
    }
    
    cout << ans << endl;
    return 0;
}