let nthroot n A =
    let rec f x =
        let m = n - 1.
        let x' = (m * x + A/x**m) / n
        match abs(x' - x) with
        | t when t < abs(x * 1e-9) -> x'
        | _ -> f x'
    f (A / double n)

[<EntryPoint>]
let main args =
    if args.Length <> 2 then
        eprintfn "usage: nthroot n A"
        exit 1
    let (b, n) = System.Double.TryParse(args.[0])
    let (b', A) = System.Double.TryParse(args.[1])
    if (not b) || (not b') then
        eprintfn "error: parameter must be a number"
        exit 1
    printf "%A" (nthroot n A)
    0