import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
class Main {
    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();

        int d = sc.nextInt();

        int[] t = new int[n];
        int[] k = new int[n];

        for (int i=0; i<n; i++) {
            t[i] = sc.nextInt();
            k[i] = sc.nextInt();
        }

        long t1 = t[0];
        long k1 = k[0]-d;

        long tt = 0l;
        long kk = 0l;
        
        for (int i=1; i<n; i++) {
            
            tt = Math.max(t1 + t[i], k1 - d + t[i]);
            kk = Math.max(k1 + k[i], t1 - d + k[i]);

            t1 = tt;
            k1 = kk;
        }
        
        System.out.println(Math.max(tt,kk));
    }
}