import java.util.*;
public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int n = sc.nextInt();
		int fare = sc.nextInt();
		long[] dpt = new long[n];
		long[] dpk = new long[n];
		dpt[0] = sc.nextInt();
		dpk[0] = sc.nextInt()-fare;
		int tokyo = 0;
		int kyoto = 0;
		for (int i=1; i<n; i++) {
			tokyo = sc.nextInt();
			kyoto = sc.nextInt();
			dpt[i] = tokyo + Math.max(dpt[i-1],dpk[i-1]-fare);
			dpk[i] = kyoto + Math.max(dpt[i-1]-fare,dpk[i-1]);
		}
		System.out.println(Math.max(dpt[n-1],dpk[n-1]));
	}
}