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


fun findAns n k =
    let
        val numerator = List.foldl
                      (fn (s, acc) =>
                          (LargeInt.fromInt s) + acc)
                      (LargeInt.fromInt 0)
                      (List.tabulate (n, fn i => (i + 1) * k))
        val denominator = ((LargeInt.fromInt n) + (LargeInt.fromInt 1)) * (LargeInt.fromInt k)
    in
        (Real.fromLargeInt numerator) / (Real.fromLargeInt) denominator
    end


val () =
    let
        val n = readInt ()
        val k = readInt ()

        val ans = findAns n k
    in
        print (Real.toString ans ^ "\n")
    end