import sugar, strutils, queues


proc nextString: string =
    result = ""
    while not endOfFile stdin:
        let nextChar = readChar stdin
        case nextChar
        of '\r':
            discard
        of "\n"[0], ' ':
            break
        else:
            add result, nextChar


proc nextInt: int =
    parseInt nextString()


let
    n, driveCst = nextInt()
    TK = lc[(nextInt(), nextInt()) | (i <- 0 ..< n), tuple[tyo, kyo: int]]

var
    earningsTyo, earningsKyo = lc[int.low | (i <- 0 .. n), int]


proc bfs: int =
    var Q = initQueue[tuple[day, pool: int, place: char]]()
    enqueue Q, (0, 0, 't')
    while Q.len != 0:
        let q = dequeue Q
        if q.day == n:
            continue

        for p in ['t', 'k']:
            var poolNow = q.pool - driveCst * int(p != q.place)
            if p == 't':
                poolNow += TK[q.day].tyo
                if earningsTyo[q.day + 1] < poolNow:
                    enqueue Q, (q.day + 1, poolNow, 't')
                    earningsTyo[q.day + 1] = poolNow
            if p == 'k':
                poolNow += TK[q.day].kyo
                if earningskyo[q.day + 1] < poolNow:
                    enqueue Q, (q.day + 1, poolNow, 'k')
                    earningsKyo[q.day + 1] = poolNow

    max(earningsTyo[n], earningsKyo[n])


proc main: void =
    echo bfs()


if isMainModule:
    main()