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#include <bits/stdc++.h>

using namespace std;

int n;
const int N = 5e4 + 3;
int ciag1[N], ciag2[N];
int kier1 = 1, kier2 = -1, wyn1, wyn2;

int zamien_kierunek(int x)
{
    if (x == 1) return -1;
    if (x == -1) return 1;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> ciag1[i];
        ciag2[i] = ciag1[i];
    }
    for (int i = 1; i <= n - 1; i++)
    {
        if (ciag1[i + 1] <= ciag1[i] && kier1 == 1)
        {
            if (i != n - 1 || i != n - 2) ciag1[i + 1] = max(ciag1[i + 2], ciag1[i]) + 1;
            else ciag1[i + 1] = ciag1[i] + 1;
            wyn1++;
        }
        if (ciag1[i + 1] >= ciag1[i] && kier1 == -1)
        {
            if (i != n - 1 || i != n - 2) ciag1[i + 1] = min(ciag1[i + 2], ciag1[i]) - 1;
            else ciag1[i + 1] = ciag1[i] - 1;
            wyn1++;
        }
        kier1 = zamien_kierunek(kier1);

        if (ciag2[i + 1] <= ciag2[i] && kier2 == 1)
        {
            if (i != n - 1 || i != n - 2) ciag2[i + 1] = max(ciag2[i + 2], ciag2[i]) + 1;
            else ciag2[i + 1] = ciag2[i] + 1;
            wyn2++;
        }
        if (ciag2[i + 1] >= ciag2[i] && kier2 == -1)
        {
            if (i != n - 1 || i != n - 2) ciag2[i + 1] = min(ciag2[i + 2], ciag2[i]) - 1;
            else ciag2[i + 1] = ciag2[i] - 1;
            wyn2++;
        }
        kier2 = zamien_kierunek(kier2);
    }
    cout << min(wyn1, wyn2);
    return 0;
}