import java.util.*;

public class Main {
	public static void main (String[] args) {
    	Scanner sc = new Scanner(System.in);
    	int n = sc.nextInt();
    	int d = sc.nextInt();
    	int prevT = sc.nextInt();
    	int prevK = sc.nextInt() - d;
    	for (int i = 1; i < n; i++) {
    	    int nextT = sc.nextInt() + Math.max(prevT, prevK - d);
    	    int nextK = sc.nextInt() + Math.max(prevT - d, prevK);
    	    prevT = nextT;
    	    prevK = nextK;
    	}
    	System.out.println(Math.max(prevT, prevK));
	}
}