using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static long NN => long.Parse(ReadLine());
    static long[] NList => ReadLine().Split().Select(long.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var c = NList;
        var (n, d) = (c[0], c[1]);
        var dpt = 0L;
        var dpk = long.MinValue / 2;
        for (var i = 0; i < n; ++i)
        {
            c = NList;
            var ndpt = Math.Max(dpt + c[0], dpk - d + c[0]);
            var ndpk = Math.Max(dpt - d + c[1], dpk + c[1]);
            dpt = ndpt;
            dpk = ndpk;
        }
        WriteLine(Math.Max(dpt, dpk));
    }
}