let f (s:string) =
    s.Split ' '
    |> Array.map int
    |> fun (a:int[]) -> (a.[0], a.[1])

type Buf = {
    mutable tm: int
    mutable km: int
    d: int
} with
    member this.max = max this.tm this.km
    member this.update(s:string) =
        let (tm, km) =
            s
            |> f
            |> fun (t:int, k:int) ->
                (t + (max this.tm (this.km - this.d)),
                 k + (max (this.tm - this.d) this.km))
        this.tm <- tm
        this.km <- km

let (n, d) = stdin.ReadLine () |> f
let (t, k) = stdin.ReadLine () |> f
let b:Buf = { tm = t; km = k - d; d = d }

seq { for _ in 1..n-1 -> stdin.ReadLine () }
    |> Seq.iter (fun s -> b.update(s))

b.max |> printfn "%d"