using System;
using System.Collections.Generic;
using static Assistant.Input;
using static Assistant.Debug;
using System.Linq;
using Assistant;

namespace ABC095D
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = ReadLong;
            var C = ReadLong;

            long[] xr = new long[N];
            long[] xl = new long[N];
            long[] vr = new long[N];
            long[] vl = new long[N];
            for (int i = 0; i < N; i++)
            {
                long index = N - i - 1;
                long x = ReadLong;
                long v = ReadLong;
                xr[i] = x;
                xl[index] = C - x;
                vr[i] = v;
                vl[index] = v;
            }

            for (int i = 0; i < N - 1; i++)
            {
                vr[i + 1] += vr[i];
                vl[i + 1] += vl[i];
            }

            long[] gr = new long[N];
            long[] gl = new long[N];
            for (int i = 0; i < N; i++)
            {
                gr[i] = vr[i] - xr[i];
                gl[i] = vl[i] - xl[i];
            }
            for (int i = 0; i < N - 1; i++)
            {
                gr[i + 1] = Math.Max(gr[i], gr[i + 1]);
                gl[i + 1] = Math.Max(gl[i], gl[i + 1]);
            }

            long ans = 0;
            for (int i = 0; i < N - 1; i++)
            {
                long[] cand = new long[4];
                cand[0] = vr[i] - 2 * xr[i] + gl[N - 2 - i];
                cand[1] = vl[i] - 2 * xl[i] + gr[N - 2 - i];

                ans = Math.Max(ans, cand.Max());
            }
            ans = Math.Max(ans, Math.Max(gr[N - 1], gl[N - 1]));

            Console.WriteLine(ans);
        }


    }


}

namespace Assistant
{
    static class Input
    {
        static List<string> line = new List<string>();
        static int index = 0;
        static String ReadNext()
        {
            if (line.Count <= index) line.AddRange(Console.ReadLine().Split());
            return line[index++];
        }
        public static int ReadInt => int.Parse(ReadNext());
        public static long ReadLong => long.Parse(ReadNext());
        public static int[] ReadInts => Console.ReadLine().Split().Select(int.Parse).ToArray();
        public static long[] ReadLongs => Console.ReadLine().Split().Select(long.Parse).ToArray();
    }

    static class Debug
    {
        static public void Draw2D(int[,] map, int mode = 1)
        {
#if DEBUG
            int W = map.GetLength(0);
            int H = map.GetLength(1);

            int[,] map2 = new int[W + 1, H + 1];
            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (i == 0 && j == 0) map2[i, j] = 0;
                    else if (i == 0) map2[i, j] = j - 1;
                    else if (j == 0) map2[i, j] = i - 1;
                    else map2[i, j] = map[i - 1, j - 1];
                }
            }

            for (int i = 0; i < W + 1; i++)
            {
                for (int j = 0; j < H + 1; j++)
                {
                    if (mode == 0) Console.Write(map2[i, j] % 10);
                    if (mode == 1) Console.Write(map2[i, j] + " ");
                }
                Console.WriteLine();
            }
            Console.WriteLine();
#endif
        }

        public static void Draw1D(int[] array, int mode = 0)
        {
#if DEBUG
            Console.WriteLine(string.Join(" ", array));
#endif
        }
    }
}


Measured memory_bytes: 34584