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

namespace yukicoder
{
	class Programs
	{
		static void Main(string[] args)
		{
			var line1 = Console.ReadLine().Split(' ').Select(c => int.Parse(c)).ToArray();
			int N = line1[0];
			int D = line1[1];
			
			int[,] pay = new int[N,2];
			for(int i = 0;i < N;i++)
			{
				var line2 = Console.ReadLine().Split(' ').Select(c => int.Parse(c)).ToArray();
				pay[i,0] = line2[0];//東京
				pay[i,1] = line2[1];//京都
			}
			
			int[,] dp = new int[N,2];
			dp[0,0] = pay[0,0];
			dp[0,1] = pay[0,1] - D;
			
			for(int i = 1;i < N;i++)//i日目dp
			{
				for(int p = 0;p < 2;p++)//今日の東京、京都更新
				{
					int maxPay = int.MinValue;
					for(int q = 0;q < 2;q++)
					{
						int todayPay  = pay[i,p] + dp[i - 1,q];
						if(p != q)todayPay -= D;
						if(maxPay <= todayPay)maxPay = todayPay;
					}
					dp[i,p] = maxPay;
				}
			}
			Console.WriteLine(Math.Max(dp[N-1,0],dp[N-1,1]));
			
		}
		
	}
	
}