using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string acsClaimNumber = "171028303";

        string apdClaimNumber = "17-1028303";

        Console.WriteLine(acsClaimNumber.ToApdFormat());

        Console.WriteLine(apdClaimNumber.ToAcsFormat());

    }       

}

​

public static class Extensions

{

    public static string ToAcsFormat(this string claimNumber)       

    {

        return claimNumber.Replace("-", string.Empty);

    }

    public static string ToApdFormat(this string claimNumber)

    {

        return Regex.Replace(claimNumber, @"^[\d]{1}", m => string.Format(@"{0}-", m.Value));

    }

}