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#include <bits/stdc++.h>

using namespace std;

int n, l = 0;
int w1, w2, wynik;
vector <int> w;
vector <int> p;

void idz(bool kierunek)
{
    bool g;

    g = kierunek;

    for (int i = 1; i < n-1; i++)
    {
        if (!g)
        {
            if (w[i] >= w[i-1] && w[i] >= w[i+1])
            {
                w[i] = min(w[i+1], w[i-1])-1;
                l++;
            }
        }

        if (g)
        {
            if (w[i] <= w[i-1] && w[i] <= w[i+1])
            {
                w[i] = max(w[i+1], w[i-1])+1;
                l++;
            }
        }

        if (!g)
            g = true;
        else
            g = false;
    }

    g = kierunek;

    for (int i = 1; i < n-1; i++)
    {
        if (!g)
        {
            if (w[i] >= w[i-1] || w[i] >= w[i+1])
            {
                w[i] = min(w[i+1], w[i-1])-1;
                l++;
            }
        }

        if (g)
        {
            if (w[i] <= w[i-1] || w[i] <= w[i+1])
            {
                w[i] = max(w[i+1], w[i-1])+1;
                l++;
            }
        }

        if (!g)
            g = true;
        else
            g = false;
    }

    if (kierunek)
        w2 = l;
    else
        w1 = l;

}

int main()
{
    int a;

    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    cin >> n;

    for (int i = 0; i < n; i++)
    {
        cin >> a;
        p.push_back(a);
        w.push_back(a);
    }

    idz(false);
    l = 0;

    for (int i = 0; i < n; i++)
        w[i] = p[i];

    idz(true);

    wynik = min(w1, w2);

    cout << wynik;

    exit(0);
}