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#include <iostream>
#include <string>
#include <algorithm>
#include <vector>

using namespace std;

int solve(int n, vector<int> arr, int sign)
{
    int res = 0;
    for (int i = 1; i < n; ++i)
    {
        if ((arr[i] - arr[i - 1]) * sign <= 0)
        {
            ++res;
            if (sign > 0)
            {
                arr[i] = arr[i - 1] + 1;
                if (i < n - 1) arr[i] = max(arr[i], arr[i + 1] + 1);
            }
            else
            {
                arr[i] = arr[i - 1] - 1;
                if (i < n - 1) arr[i] = min(arr[i], arr[i + 1] - 1);
            }
        }
        sign *= -1;
    }
    return res;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int n;
    cin >> n;

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

    cout << min(solve(n, arr, 1), solve(n, arr, -1));
}