let isPal10 str =

    let str = System.Convert.ToInt32(str, 2) |> string

    let mutable pal = true

    for i in 0 .. str.Length / 2 do

        if str.[i] <> str.[str.Length - 1 - i] then

            pal <- false

    pal

seq {

    yield "1"

    let m2 = ref (seq [ "" ])

    let m1 = ref (seq [ "0"; "1" ])

    for i in 2 .. 20 do

        let p = Seq.collect (fun p -> [ "0" + p + "0"; "1" + p + "1" ]) !m2

        m2 := !m1

        m1 := p

        yield! p |> Seq.filter (fun s -> s.[0] <> '0' && isPal10 s)

}

|> Seq.map (fun s -> System.Convert.ToInt32(s, 2))

|> Seq.sum

|> printfn "%d"