#include<bits/stdc++.h>
using namespace std;
int a[2010]={0};
int b[2010]={0};
int main()
{
    freopen("task.in","r",stdin);
    freopen("task.out","w",stdout);
    int a1,b1,ans,n,i;
    cin>>n;
    a1=0;b1=0;ans=0;
    for(i=1;i<=n;i++)
    {
        cin>>a[i]>>b[i];
    }
    for(i=1;i<=n;i++)
    {
        if(max(a[i]+a1,b[i+1]+b1)>max(b[i]+b1,a[i+1]+a1))
        {
            b1=b1+b[i];
            //cout<<"b1:"<<b1<<endl;
        }
        else
        {
            a1=a1+a[i];
            //cout<<"a1:"<<a1<<endl;
        }
        if(a1>b1)
        {
            ans=ans+b1;
            a1=a1-b1;
            b1=0;
        }
        else
        {
            if(a1<b1)
            {
                ans=ans+a1;
                b1=b1-a1;
                a1=0;
            }
        }
    }
    cout<<max(a1,b1)+ans;
    return 0;
}