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#include <bits/stdc++.h>

using namespace std;

int main()
{
    int n;
    cin >> n;
    vector <int> inp(n+1);
    vector <int> dupl(n+1);
    for(int i=0; i<n; ++i)
    {
        cin >> inp[i];
    }
    inp[n] = inp[n-1];

    // assend
    int resas=0, resdes=0;
    dupl = inp;
     for(int i=0; i<n-1; ++i)
    {
        if(i%2 == 0)
        {
            if(inp[i+1] <= inp[i])
            {
                resas++;
                inp[i+1] = max(inp[i], inp[i+2])+1;
            }
        }
        else if(inp[i+1] >= inp[i])
        {
            resas++;
            inp[i+1] = min(inp[i], inp[i+2])-1;

        }
   //   cerr << inp[i+1];
    }
   // cerr << "\n";
    //dessend
    inp = dupl;

    for(int i=0; i<n-1; ++i)
    {
        if(i%2 == 1)
        {
            if(inp[i+1] <= inp[i])
            {
                resdes++;
                inp[i+1] = max(inp[i], inp[i+2])+1;
            }
        }
        else if(inp[i+1] >= inp[i])
        {
            resdes++;
            inp[i+1] = min(inp[i], inp[i+2])-1;
        }
    //  cerr << inp[i+1];
    }
    //cerr << "\n";
    cout << min(resas, resdes);

    return 0;
}