#!/usr/bin/env perl

use strict;
use warnings;

my ($n, $d) = split / /, <>;
chomp $d;

my @dp;
$dp[0][0] = 0;
$dp[0][1] = -999999999999999999;
for my $i (1..110) {
  for my $j (0..1) {
    $dp[$i][$j] = -1;
  }
}
for (my $i = 0; $i < $n; $i++) {
  my ($t, $k) = split / /, <>;
  chomp $k;

  my ($x, $y) = ($dp[$i][0] + $t, $dp[$i][1] + $t - $d);
  $dp[$i + 1][0] = $x if $x >= $y;
  $dp[$i + 1][0] = $y if $x < $y;

  my ($z, $w) = ($dp[$i][1] + $k, $dp[$i][0] + $k - $d);
  $dp[$i + 1][1] = $z if $z >= $w;
  $dp[$i + 1][1] = $w if $z < $w;
}

if ($dp[$n][0] >= $dp[$n][1]) {
  print $dp[$n][0];
} else {
  print $dp[$n][1];
}