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#include<bits/stdc++.h>
using namespace std;
int tab[50005];
int tab1[50005];
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n,w1=0,w2=0;
    cin>>n;
    for(int i=0;i<n;i++)
    {
        cin>>tab[i];
        tab1[i]=tab[i];

    }
    for(int i=1;i<n;i++)
    {

        if(i%2==0)
        {
            if(tab[i-1]<=tab[i])
            {
                w1++;
                tab[i]=min(tab[i-1],tab[i+1])-1;
                //cout<<"=";
                //cout<<tab[i]<<" ";
            }
            if(tab1[i-1]>=tab1[i])
            {
                w2++;
                tab1[i]=max(tab1[i-1],tab1[i+1])+1;
                //cout<<"+";
                //cout<<tab1[i]<<" ";
            }
        }
        else
        {
            if(tab[i-1]>=tab[i])
            {
                w1++;
                tab[i]=max(tab[i-1],tab[i+1])+1;
                //cout<<";";
                //cout<<tab[i]<<" ";
            }
            if(tab1[i-1]<=tab1[i])
            {
                w2++;
                tab1[i]=min(tab1[i-1],tab1[i+1])-1;
                //cout<<":";
                //cout<<tab1[i]<<" ";
            }
        }
        //cout<<endl;
    }
    cout<<min(w1,w2);



}