import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        int[][] dp;

        public void solve(int testNumber, Scanner in, PrintWriter out) {
            final int n = in.nextInt();
            final int d = in.nextInt();

            final int[] t = new int[n];
            final int[] k = new int[n];
            for (int i = 0; i < n; i++) {
                t[i] = in.nextInt();
                k[i] = in.nextInt();
            }

            dp = new int[n][2]; // dp[day][tokyo or kyoto]
            dp[0][0] = t[0];
            dp[0][1] = k[0] - d;

            for (int i = 1; i < n; i++) {
                dp[i][0] = Math.max(dp[i - 1][0] + t[i], dp[i - 1][1] + t[i] - d);
                dp[i][1] = Math.max(dp[i - 1][0] + k[i] - d, dp[i - 1][1] + k[i]);
            }

            out.println(Math.max(dp[n - 1][0], dp[n - 1][1]));
        }

    }
}