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#include <iostream>

using namespace std;

int main()
{
    int n;
    cin >> n;

    int a[n];

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

    int changes = 0;

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

cout << changes << endl;
}