fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


fun intString n =
    if 0 <= n
    then Int.toString n
    else "-" ^ Int.toString (abs n)


fun findAns a b c d e =
    let
        fun findAnsAux deliciousness calories bacon maxHappiness =
            if c <= bacon then maxHappiness
            else findAnsAux (deliciousness + d) (calories + e) (bacon + 1) (Int.max((deliciousness + d) - (calories + e), maxHappiness))
    in
        findAnsAux a b 0 (a - b)
    end


val () =
    let
        val a = readInt ()
        val b = readInt ()
        val c = readInt ()
        val d = readInt ()
        val e = readInt ()

        val ans = findAns a b c d e
    in
        print (intString ans ^ "\n")
    end