package yuki172;

import java.util.Scanner;

public class D {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long D = sc.nextLong();
		long[] T = new long[N];
		long[] K = new long[N];
		long[] Tans = new long[N];
		long[] Kans = new long[N];
 		for(int i=0; i<N; i++) {
			T[i] = sc.nextLong();
			K[i] = sc.nextLong();
		}
 		sc.close();
		Tans[0] = T[0];
		Kans[0] = K[0] - D;
 		for(int i=1; i<N; i++) {
 			Tans[i] = T[i] + max(Tans[i-1], Kans[i-1]-D);
 			Kans[i] = K[i] + max(Tans[i-1]-D, Kans[i-1]);
 		}
		System.out.println(max(Tans[N-1], Kans[N-1]));
	}
	
	private static long max(long a, long b) {
		if(a > b) {
			return a;
		} else {
			return b;
		}
	}

}