using System;
using System.Collections.Generic;
using System.Linq;
using BitArray = System.Collections.BitArray;
using BigInteger = System.Numerics.BigInteger;
using TextReader = System.IO.TextReader;
using System.Text;

namespace AtCoderProject
{
    public class Program
    {
        public object Calc()
        {
            var N = consoleReader.Int;
            var C = consoleReader.Long;
            var sushis = consoleReader.Repeat(N).Select(cr => new Sushi { x = cr.Long, v = cr.Long }).ToArray();

            var leftSum = new long[N];
            var rightSum = new long[N];
            var leftSum2 = new long[N];
            var rightSum2 = new long[N];
            var leftSum2m = new long[N];
            var rightSum2m = new long[N];
            leftSum[N - 1] = sushis[N - 1].v - (C - sushis[N - 1].x);
            leftSum2[N - 1] = leftSum[N - 1] - (C - sushis[N - 1].x);
            rightSum[0] = sushis[0].v - sushis[0].x;
            rightSum2[0] = rightSum[0] - sushis[0].x;
            long leftMax = leftSum2m[N - 1] = Math.Max(0, leftSum2[N - 1]);
            long rightMax = rightSum2m[0] = Math.Max(0, rightSum2[0]);
            for (int i = 1; i < N; i++)
            {
                var leftDistance = sushis[N - i].x - sushis[N - i - 1].x;
                leftSum[N - i - 1] = sushis[N - i - 1].v + leftSum[N - i] - leftDistance;
                leftSum2[N - i - 1] = leftSum[N - i - 1] - (C - sushis[N - i - 1].x);
                if (leftMax < leftSum2[N - i - 1])
                {
                    leftMax = leftSum2[N - i - 1];
                }
                leftSum2m[N - i - 1] = leftMax;

                var rightDistance = sushis[i].x - sushis[i - 1].x;
                rightSum[i] = sushis[i].v + rightSum[i - 1] - rightDistance;
                rightSum2[i] = rightSum[i] - sushis[i].x;
                if (rightMax < rightSum2[i])
                {
                    rightMax = rightSum2[i];
                }
                rightSum2m[i] = rightMax;
            }

            long max = new[] { leftSum[0], rightSum[N - 1], 0 }.Max();
            for (int i = 1; i < N; i++)
            {
                max = Math.Max(Math.Max(
                    leftSum[i] + rightSum2m[i - 1],
                    rightSum[N - i - 1] + leftSum2m[N - i]
                    ), max);
            }

            return max;
        }

        struct Sushi
        {
            public long x;
            public long v;
        }

        #region いつもの
        private readonly ConsoleReader consoleReader;
        public Program(ConsoleReader consoleReader) { this.consoleReader = consoleReader; }
        static void Main() => Console.WriteLine(new Program(new ConsoleReader(Console.In)).Calc()); static string AllLines<T>(IEnumerable<T> source) => string.Join("\n", source);
    }
#pragma warning disable
    public class ConsoleReader
    {
        private string[] ReadLineSplit() => textReader.ReadLine().Split(Array.Empty<char>(), StringSplitOptions.RemoveEmptyEntries);
        private string[] line = Array.Empty<string>();
        private int linePosition;
        private TextReader textReader;
        public ConsoleReader(TextReader tr) { textReader = tr; }
        public int Int => int.Parse(String);
        public long Long => long.Parse(String);
        public double Double => double.Parse(String);
        public string String
        {
            get
            {
                if (linePosition >= line.Length)
                {
                    linePosition = 0;
                    line = ReadLineSplit();
                }
                return line[linePosition++];
            }
        }
        public class SplitLine
        {
            private string[] splited;
            public SplitLine(ConsoleReader cr) { splited = cr.ReadLineSplit(); cr.line = Array.Empty<string>(); }
            public int[] Int => String.Select(x => int.Parse(x)).ToArray();
            public int[] Int0 => String.Select(x => int.Parse(x) - 1).ToArray();
            public long[] Long => String.Select(x => long.Parse(x)).ToArray();
            public double[] Double => String.Select(x => double.Parse(x)).ToArray();
            public string[] String => splited;
        }
        public SplitLine Split => new SplitLine(this);
        public class RepeatReader : IEnumerable<ConsoleReader>
        {
            ConsoleReader cr; int count;
            public RepeatReader(ConsoleReader cr, int count) { this.cr = cr; this.count = count; }
            public IEnumerator<ConsoleReader> GetEnumerator() => Enumerable.Repeat(cr, count).GetEnumerator();
            System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator() => GetEnumerator();

            public IEnumerable<string> String => this.Select(cr => cr.String);
            public IEnumerable<int> Int => this.Select(cr => cr.Int);
            public IEnumerable<int> Int0 => this.Select(cr => cr.Int - 1);
            public IEnumerable<long> Long => this.Select(cr => cr.Long);
            public IEnumerable<double> Double => this.Select(cr => cr.Double);
        }
        public RepeatReader Repeat(int count) => new RepeatReader(this, count);
    }
    #endregion
}

Measured memory_bytes: 27328