using System;

static class Program{
    static void Main(){
        new Magatro().Solve();
    }
}

class Magatro{
    public void Solve(){
        int n,d;
        int[]t,k;;
        string[]line = Console.ReadLine().Split(' ');
        n = int.Parse(line[0]);
        d = int.Parse(line[1]);
        t = new int[n];
        k = new int[n];
        for(int i =0;i<n;i++){
            line = Console.ReadLine().Split(' ');
            t[i]= int.Parse(line[0]);
            k[i]=int.Parse(line[1]);
        }
        int tt = t[0],kk = k[0]-d;
        for(int i =1;i<n;i++){
            int nt = Math.Max(tt,kk-d)+t[i];
            int nk = Math.Max(kk,tt-d)+k[i];
            tt = nt;
            kk = nk;
        }
        Console.WriteLine(Math.Max(tt,kk));
    }
}