let ``No.18 うーさー暗号`` () =
    let f n (c:char) = string <| (char)((((int)c % 65 + 26) - ((n+1)%26)) % 26 + 65)
    stdin.ReadLine()
    |> Seq.mapi f
    |> Seq.reduce (+)
    |> printfn "%s"
``No.18 うーさー暗号`` ()