How to convert from String to Int in Json.Decoder

... and to answer myself :) I found the solution in this Flickr example

decodeData : Json.Decoder (Id, String)
decodeData =
  let number =
    Json.oneOf [ Json.int, Json.customDecoder Json.string String.toInt ]
  in
    Json.at ["data", "0"]
      <| Json.object2 (,)
        ("id" := number)
        ("label" := Json.string)