using System;
using System.IO;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;

partial class Solver
{

    public void Run()
    {
        var N = ni();
        var C = nl();
        var x = new long[N];
        var v = new long[N];

        for (int i = 0; i < N; i++)
        {
            x[i] = nl();
            v[i] = nl();
        }

        long ans = 0;

        ans = Math.Max(ans, v.Sum() - C); // 1-loop

        var leftMax = new long[N];
        var leftReturnMax = new long[N];
        long leftSum = 0;
        var rightMax = new long[N];
        var rightReturnMax = new long[N];
        long rightSum = 0;

        for (int i = 0; i < N; i++)
        {
            leftSum += v[i];
            if (i - 1 >= 0)
            {
                leftMax[i] = leftMax[i - 1];
                leftReturnMax[i] = leftReturnMax[i - 1];
            }
            leftMax[i] = Math.Max(leftMax[i], leftSum - x[i]);
            leftReturnMax[i] = Math.Max(leftReturnMax[i], leftSum - x[i] * 2);
        }
        for (int i = N - 1; i >= 0; i--)
        {
            rightSum += v[i];
            if (i + 1 < N)
            {
                rightMax[i] = rightMax[i + 1];
                rightReturnMax[i] = rightReturnMax[i + 1];
            }
            rightMax[i] = Math.Max(rightMax[i], rightSum - (C - x[i]));
            rightReturnMax[i] = Math.Max(rightReturnMax[i], rightSum - (C - x[i]) * 2);
        }


        for (int i = 0; i < N; i++)
        {
            if (i + 1 < N)
            {
                ans = Math.Max(ans, leftReturnMax[i] + rightMax[(i + 1)]);
                ans = Math.Max(ans, leftMax[i] + rightReturnMax[(i + 1)]);
            }

            ans = Math.Max(ans, rightMax[i]);
            ans = Math.Max(ans, leftMax[i]);
        }

        cout.WriteLine(ans);
    }
}

// PREWRITEN CODE BEGINS FROM HERE
partial class Solver : Scanner
{
    public static void Main(string[] args)
    {
        new Solver(Console.In, Console.Out).Run();
    }

    private readonly TextReader cin;
    private readonly TextWriter cout;

    public Solver(TextReader reader, TextWriter writer)
        : base(reader)
    {
        this.cin = reader;
        this.cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer)
    {
    }

    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
}

public class Scanner
{
    private TextReader Reader;
    private Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In)
    {
    }

    public Scanner(TextReader reader)
    {
        this.Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size)
    {
        var array = new string[size];
        for (int i = 0; i < size; i++) array[i] = Next();
        return array;
    }
    public int[] NextIntArray(int size)
    {
        var array = new int[size];
        for (int i = 0; i < size; i++) array[i] = NextInt();
        return array;
    }

    public long[] NextLongArray(int size)
    {
        var array = new long[size];
        for (int i = 0; i < size; i++) array[i] = NextLong();
        return array;
    }

    public string Next()
    {
        if (TokenQueue.Count == 0)
        {
            if (!StockTokens()) throw new InvalidOperationException();
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext()
    {
        if (TokenQueue.Count > 0)
            return true;
        return StockTokens();
    }

    private bool StockTokens()
    {
        while (true)
        {
            var line = Reader.ReadLine();
            if (line == null) return false;
            var tokens = line.Trim().Split(" ".ToCharArray(), StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) continue;
            foreach (var token in tokens)
                TokenQueue.Enqueue(token);
            return true;
        }
    }
}


Measured memory_bytes: 19916