using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static string InputPattern = "InputX";

    static List<string> GetInputList()
    {
        var WillReturn = new List<string>();

        if (InputPattern == "Input1") {
            WillReturn.Add("2 10000");
            WillReturn.Add("20000 32000");
            WillReturn.Add("24000 23000");
            //45000
        }
        else if (InputPattern == "Input2") {
            WillReturn.Add("2 10000");
            WillReturn.Add("42000 50000");
            WillReturn.Add("58000 56000");
            //100000
        }
        else if (InputPattern == "Input3") {
            WillReturn.Add("3 10");
            WillReturn.Add("42000 50000");
            WillReturn.Add("58000 56000");
            WillReturn.Add("40000 45000");
            //152970
        }
        else if (InputPattern == "Input4") {
            WillReturn.Add("5 3000");
            WillReturn.Add("10000 10000");
            WillReturn.Add("20000 23000");
            WillReturn.Add("30000 38000");
            WillReturn.Add("44000 40000");
            WillReturn.Add("54000 56000");
            //164000
        }
        else {
            string wkStr;
            while ((wkStr = Console.ReadLine()) != null) WillReturn.Add(wkStr);
        }
        return WillReturn;
    }

    struct TKInfoDef
    {
        internal int T;
        internal int K;
    }

    static void Main()
    {
        List<string> InputList = GetInputList();

        int[] wkArr = { };
        Action<string> SplitAct = pStr =>
            wkArr = pStr.Split(' ').Select(X => int.Parse(X)).ToArray();

        SplitAct(InputList[0]);
        int D = wkArr[1];

        var TKInfoList = new List<TKInfoDef>();
        foreach (string EachStr in InputList.Skip(1)) {
            SplitAct(EachStr);
            TKInfoList.Add(new TKInfoDef() { T = wkArr[0], K = wkArr[1] });
        }

        //収入合計[Is東京]なDP表
        var PrevDP = new Dictionary<bool, int>();
        PrevDP[true] = 0;

        foreach (TKInfoDef EachTKInfo in TKInfoList) {
            var CurrDP = new Dictionary<bool, int>();

            Action<bool, int> UpdateAct = (pKey, pValue) =>
            {
                if (CurrDP.ContainsKey(pKey) && CurrDP[pKey] >= pValue)
                    return;
                CurrDP[pKey] = pValue;
            };

            foreach (var EachPair in PrevDP) {
                //東京で働く場合
                if (EachPair.Key)
                    UpdateAct(true, EachPair.Value + EachTKInfo.T);
                else
                    UpdateAct(true, EachPair.Value + EachTKInfo.T - D);

                //京都で働く場合
                if (EachPair.Key)
                    UpdateAct(false, EachPair.Value + EachTKInfo.K - D);
                else
                    UpdateAct(false, EachPair.Value + EachTKInfo.K);
            }
            PrevDP = CurrDP;
        }
        Console.WriteLine(PrevDP.Values.Max());
    }
}