using static System.Math;
using System.Linq;
using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var a = new long[n];
        for (int i = 0; i < n; i++)
        {
            line = Console.ReadLine().Trim().Split(' ');
            var w = Array.ConvertAll(line, long.Parse);
            a[i] = w.Sum();
        }
        getAns(n, a);
    }
    static void getAns(int n, long[] a)
    {
        var preT = a[0];
        var preF = 0L;
        for (int i = 1; i < n; i++)
        {
            var wT = preF + a[i];
            var wF = preT - a[i];
            wT = Max(wT, preT);
            wF = Max(wF, preF);
            preF = wF;
            preT = wT;
        }
        Console.WriteLine(Max(preT, preF));
    }
}