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#include <iostream>
using namespace std;

int n;
int sounds[50000];
int sounds1[50000];
int sounds2[50000];

int FindNumber1(bool higher, int index)
{
    if(higher)
    {
        return max(sounds1[index - 1], sounds1[index + 1]) + 1;
    }
    else
    {
        return min(sounds1[index - 1], sounds1[index + 1]) - 1;
    }
}

int FindNumber2(bool higher, int index)
{
    if(higher)
    {
        return max(sounds2[index - 1], sounds2[index + 1]) + 1;
    }
    else
    {
        return min(sounds2[index - 1], sounds2[index + 1]) - 1;
    }
}

int Count()
{
    int sol1 = 0, sol2 = 0;

    for(int i = 1; i < n; i++)
    {
        if(i % 2 == 0)
        {
            if(sounds1[i] >= sounds1[i - 1])
            {
                sounds1[i] = FindNumber1(false, i);
                sol1++;
            }

            if(sounds2[i] <= sounds2[i - 1])
            {
                sounds2[i] = FindNumber2(true, i);
                sol2++;
            }
        }

        if(i % 2 == 1)
        {
            if(sounds1[i] <= sounds1[i - 1])
            {
                sounds1[i] = FindNumber1(true, i);
                sol1++;
            }

            if(sounds2[i] >= sounds2[i - 1])
            {
                sounds2[i] = FindNumber2(false, i);
                sol2++;
            }
        }
    }

    if(sol1 <= sol2)
    {
        return sol1;
    } else
    {
        return sol2;
    }
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    
    cin >> n;

    for (int i = 0; i < n; i++)
    {
        cin >> sounds[i];
        sounds1[i] = sounds[i];
        sounds2[i] = sounds[i];
    }

    cout << Count();
}