import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n=scanner.nextInt();
		int d=scanner.nextInt();
		int t[]=new int[n+1];
		int k[]=new int[n+1];
		for(int i=1;i<=n;i++) {
			t[i]=scanner.nextInt();
			k[i]=scanner.nextInt();
		}
		int dp[][]=new int[n+1][2];
		dp[0][0]=0;
		dp[0][1]=-d;
		for(int i=1;i<=n;i++) {
			dp[i][0]=Math.max(dp[i-1][0], dp[i-1][1]-d)+t[i];
			dp[i][1]=Math.max(dp[i-1][0]-d, dp[i-1][1])+k[i];
		}
		System.out.println(Math.max(dp[n][0], dp[n][1]));
	}
}