import java.util.Scanner;

public class No561 {
	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();
		}
		int tmax[] = new int[N];
		int kmax[] = new int[N];
		tmax[0] = t[0];
		kmax[0] = k[0] - D;
		for(int i = 1;i < N;i++) {
			tmax[i] = Math.max(tmax[i-1] + t[i],kmax[i-1] + t[i] - D);
			kmax[i] = Math.max(kmax[i-1] + k[i],tmax[i-1] + k[i] - D);
		}
		System.out.println(Math.max(tmax[N-1], kmax[N-1]));
	}
}