Erlang Ring problem in F#

Ring.fsx

open System

type Agent<'a> = MailboxProcessor<'a>
type Msg = Step of int | End

let getRingHead n =
    let lastNode =
        Agent.Start(fun inbox ->
            let rec loop (time : DateTime) = async {
                    let! m = inbox.Receive()
                    if m = End then
                        printfn "%.2fs" (DateTime.Now - time).TotalSeconds
                    return! loop time
                }
            loop DateTime.Now)
            
    let getPrevAgent (agent : _ Agent) =
        Agent.Start(fun inbox -> async {
                while true do                    
                    let! m = inbox.Receive()
                    agent.Post m
            })

    Seq.unfold (fun a -> Some(a, getPrevAgent a)) lastNode |> Seq.nth n

let run nodeCount messageCount =
    let post = (getRingHead nodeCount).Post
    Seq.init messageCount Step |> Seq.iter post
    post End

run 100 100000