import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int D = scan.nextInt();
		int[]T = new int[N];
		int[]K = new int[N];
		for(int i = 0; i < N; i++) {
			T[i] = scan.nextInt();
			K[i] = scan.nextInt();
		}
		scan.close();
		int[][]dp = new int[N + 1][2];
		dp[1][0] = T[0];
		dp[1][1] = K[0] - D;
		for(int i = 2; i <= N; i++) {
			dp[i][0] = Math.max(dp[i - 1][0] + T[i - 1], dp[i - 1][1] - D + T[i - 1]);
			dp[i][1] = Math.max(dp[i - 1][0] - D + K[i - 1], dp[i - 1][1] + K[i - 1]);
		}
		System.out.println(Math.max(dp[N][0], dp[N][1]));
	}
}