import strutils, sequtils


proc nextStr(): string =
  while not stdin.endOfFile:
    let c = stdin.readChar
    if c == '\r': continue
    if c in [' ', '\n']: break
    result.add(c)


proc main(): void =
  let
    n, d = parseInt(nextStr())
  var
    a = newSeqWith[n+1, [0, 0]]
    ans, p = 0

  for i in 0..<n:
    let t, k = parseInt(nextStr())
    a[i] = [t, k]

  for i in 0..<n:
    let
      noMove = a[i][p] + a[i+1][p]
      move1 = a[i][(p+1) mod 2] + a[i+1][(p+1) mod 2] - d
      move2 = a[i][(p+1) mod 2] + a[i+1][p] - 2*d
    if noMove < move1 or noMove < move2:
      p = (p+1) mod 2
      ans -= d
    ans += a[i][p]
  echo ans


when isMainModule: main()