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[][] dp = new int[2][n + 1];
		dp[1][0] = Integer.MIN_VALUE / 10;
		for (int i = 1; i <= n; i++) {
		    int a = sc.nextInt();
		    int b = sc.nextInt();
		    dp[0][i] = Math.max(dp[0][i - 1], dp[1][i - 1] - d) + a;
		    dp[1][i] = Math.max(dp[0][i - 1] - d, dp[1][i - 1]) + b;
		}
		System.out.println(Math.max(dp[0][n], dp[1][n]));
	}
}