#include<iostream>
#include<algorithm>
using namespace std;
int N;
main()
{
	cin>>N;
	int A=0,B=-1e9;
	for(int i=0;i<N;i++)
	{
		int p,a,ta,tb;
		cin>>p>>a;
		ta=max(A+p,B+p+p);
		tb=max(A+a,B+a+a);
		A=ta;
		B=tb;
	}
	cout<<max(A,B)<<endl;
}