using System;

public class Hello
{
    public static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = int.Parse(line[0]);
        var d = int.Parse(line[1]);
        var tk = new int[n, 2];
        for (int i = 0; i < n; i++)
        {
            line = Console.ReadLine().Trim().Split(' ');
            tk[i, 0] = int.Parse(line[0]);
            tk[i, 1] = int.Parse(line[1]);
        }
        var dp = new int[n + 1, 2];
        dp[0, 1] = -d;
        for (int i = 0; i < n; i++)
            for (int j = 0; j < 2; j++)
            {
                dp[i + 1, j] = Math.Max(dp[i + 1, j], dp[i, j] + tk[i, j]);
                dp[i + 1, 1 - j] = Math.Max(dp[i + 1, 1 - j], dp[i, j] + tk[i, 1 - j] - d);
            }
        Console.WriteLine(Math.Max(dp[n, 0], dp[n, 1]));
    }
}