import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    long d = sc.nextLong();
    long[][] dp = new long[n][2];
    long t0 = sc.nextLong();
    long k0 = sc.nextLong();
    dp[0][0] = t0;
    dp[0][1] = k0 - d;
    for(int i = 1; i < n; i++) {
      long t = sc.nextLong();
      long k = sc.nextLong();
      long tokyo = Math.max(dp[i - 1][0] + t, dp[i - 1][1] + t - d);
      long kyoto = Math.max(dp[i - 1][0] + k - d, dp[i - 1][1] + k);
      dp[i][0] = tokyo;
      dp[i][1] = kyoto;
    }
    System.out.println(Math.max(dp[n - 1][0], dp[n - 1][1]));
  }
}