using System;

public class Program

{

    public static void Main()

    {

    }

public static string FormatPhoneNumber(string phoneNumber){

        string intString = "";

        foreach (char c in phoneNumber)

        {

            if (Char.IsDigit(c))

            {

                intString += c;

            }

            if (intString.Length == 9)

            {

                break;

            }

        }

        string result = "";

        int i = 0;

        do 

        {

            for (int j = 0; j < 3 && i < intString.Length; j++) {

                result += intString[i];

                i++;

            }

            result += "-";

​

           if (i < intString.Length) {

            result += "-";

           }

        }

        while (i < intString.Length);

​

        return result;

}

}