import strutils, sequtils, algorithm, future, math

type int = int64

when isMainModule:
  var
    input = stdin.readline.split.map(parseint)
    (n, d) = (input[0], input[1])
    dp = newSeqWith(n+1, newSeq[int](2))
  dp[0][1] = -d
  for i in 1..n:
    var
      input = stdin.readline.split.map(parseint)
      (a, b) = (input[0], input[1])
    dp[i][0] = max(dp[i-1][0] + a, dp[i-1][1] + b - d)
    dp[i][1] = max(dp[i-1][0] + a - d, dp[i-1][1] + b)
  echo max(dp[n][0], dp[n][1])