let main _ = 
    let rec fib n = if n < 2 then n else fib(n - 1) + fib(n - 2)
    let w = Stopwatch.StartNew()
    for n in 0..39 do printfn "fib(%d) = %d" n (fib n)
    printfn "%O elapsed." w.Elapsed
    Console.ReadKey() |> ignore
    0