import std.algorithm, std.conv, std.range, std.stdio, std.string;
import std.container; // SList, DList, BinaryHeap
import std.typecons;  // Tuple, Nullable, BigFlags
import std.math;      // math functions
import std.numeric;   // gcd, fft
import std.bigint;    // BigInt
import std.random;    // random
import std.bitmanip;  // BitArray
import core.bitop;    // bit operation
import std.regex;     // RegEx
import std.uni;       // unicode

void main()
{
  auto rd = readln.split, n = rd[0].to!size_t, d = rd[1].to!int;
  auto t = new int[](n), k = new int[](n);
  foreach (i; 0..n) {
    auto rd2 = readln.split.to!(int[]);
    t[i] = rd2[0];
    k[i] = rd2[1];
  }

  auto dp1 = new int[](n), dp2 = new int[](n);
  dp1[0] = t[0];
  dp2[0] = k[0] - d;

  foreach (i; 1..n) {
    dp1[i] = max(dp1[i-1]+t[i],   dp2[i-1]+t[i]-d);
    dp2[i] = max(dp1[i-1]+k[i]-d, dp2[i-1]+k[i]);
  }

  writeln(max(dp1[$-1], dp2[$-1]));
}