#include <fstream>

using namespace std;

int a[100001],b[100001];
short int n;
unsigned long long sum;
int main()
{
    ifstream fin("hacker3.in");
    ofstream fout("hacker3.out");
    fin>>n;
    unsigned short int n1=n/2,n2=n1+n%2;
    for(short int i=1;i<=n1;i++)
    {
        fin>>a[i]>>b[i];
    }

    for(short int i=1;i<=n2;i++)
    {
        fin>>a[i+n1]>>b[i+n1];
    }


    for(short int i=n2;i>=1;i--)
    {
         if(sum*2+a[i+n1]>=sum+b[i+n1])
        {
            sum=sum+b[i+n1];
        }
        else
        {
            sum=sum*2+a[i+n1];
        }
    }

    for(short int i=n1;i>=1;i--)
    {
         if(sum*2+a[i]>=sum+b[i])
        {
            sum=sum+b[i];
        }
        else
        {
            sum=sum*2+a[i];
        }
    }

    fout<<sum;
    return 0;
}