let ``No.18 うーさー暗号``() =
  stdin.ReadLine()
  |> fun args ->
    args
    |> Seq.mapi (fun i c  ->
      ((int c) - (i + 1) + 65 ) % 26 + 65 |> char)
    |> fun args -> System.String.Concat args
    |> stdout.WriteLine

``No.18 うーさー暗号``()