let ``No.791 うし数列``() =

  let s = stdin.ReadLine()
  if not (s.StartsWith("1") && s.EndsWith("3"))
    then -1
    else s
         |> Seq.countBy (fun c -> c)
         |> fun arg ->
          if (Seq.length arg > 2 || 
              arg 
              |> Seq.tryFind (fun (a,b) -> a = '1')  
              |> fun arg -> arg.IsSome && snd(arg.Value) > 1)
            then -1
            else arg 
                |> Seq.find (fun (k , v) -> '3' = k )
                |> fun t -> snd(t)
  |> stdout.WriteLine              
  ()
``No.791 うし数列``()