Web Forms :: Add Digits To A Number With 2 Decimals?

I've build this function:

I like to convert 1234,222 to:

1.234,22

But what happends below is he converts it to 1.234, so verything after the , is gone. How can i change this function so it works correctly?

Also if the number is 1.234,248 the result should be 1.234,25 , so just 2 numbers after the ,

public static string AddDigits(string source, int digitaldigits)
{
System.Globalization.NumberFormatInfo format = new System.Globalization.NumberFormatInfo();
format.NumberGroupSeparator = ".";
format.NumberDecimalDigits = digitaldigits;
decimal i = decimal.Parse("1234,222");//decimal.Parse(source);
//int i = int.Parse(original);
string str = i.ToString("N0", format);
return str;
}