package yukicoder;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.Scanner;

public class N561 {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int d = sc.nextInt();
		int i, a, b, x;
		ArrayList<Integer> works = new ArrayList<Integer>();
		LinkedList<Integer> visited = new LinkedList<Integer>();
		LinkedList<Integer> queue = new LinkedList<Integer>();
		for(i = 0;i < n;++i){
			works.add(sc.nextInt());
			works.add(sc.nextInt());
			visited.add(0);
			visited.add(0);
		}
		queue.add(0);
		queue.add(1);
		visited.set(0, works.get(0));
		visited.set(1, works.get(1) - d);
		while(queue.size() > 0){
			i = queue.poll();
			x = visited.get(i);
			
			if(i % 2 == 0){
				a = i + 3;
				b = i + 2;
			}else{
				a = i + 1;
				b = i + 2;
			}
			
			if(a < n*2 && works.get(a) + x - d> visited.get(a)){
				visited.set(a, works.get(a) + x - d);
				queue.add(a);
			}
			if(b < n*2 && works.get(b) + x > visited.get(b)){
				visited.set(b, works.get(b) + x);
				queue.add(b);
			}
		}
		
		System.out.println(Collections.max(visited));
	}
}