N, D = gets.split.map(&:to_i)
Q = N.times.map { gets.split.map(&:to_i) }

dp = Array.new(N + 1) { Array.new(2, 0) }

Q.each.with_index(1) do |(t, k), i|
  dp[i][0] = [dp[i - 1][0] + t, dp[i - 1][1] + t - D].max

  if i == 1
    dp[i][1] = dp[i - 1][0] + k - D
  else
    dp[i][1] = [dp[i - 1][0] + k - D, dp[i - 1][1] + k].max
  end
end

puts dp[N].max