open System
open System.Text

let separateStr (arg:string) =
  let rec splitStr (acc:StringBuilder) (argStr:StringBuilder) =
    match argStr.Length with
    | 0 -> ""
    | 1 | 2 | 3 -> argStr.ToString() + acc.ToString()
    | _ ->  acc.Insert(0, argStr.Chars (argStr.Length-1) )
            acc.Insert(0, argStr.Chars (argStr.Length-2) )
            acc.Insert(0, argStr.Chars (argStr.Length-3) )
            acc.Insert(0, ',' )
            argStr.Length <- argStr.Length-3
            splitStr  acc argStr 
             
  splitStr (new StringBuilder()) (new StringBuilder(arg))
  
Console.ReadLine() 
|> separateStr  
|> printfn "%s"