Adjacent number algorithm grouper

By which I mean this:

Given the input set of numbers:

1,2,3,4,5 becomes "1-5".

1,2,3,5,7,9,10,11,12,14 becomes "1-3, 5, 7, 9-12, 14"

This is the best I managed to come up with: [C#]

Which feels a little sloppy to me, so the question is, is there somehow more readable and/or elegant solution to this?

public static string[] FormatInts(int[] ints)
{
    if (ints == null)
        throw new ArgumentNullException("ints"); // hey what are you doing?

    if (ints.Length == 0)
        return new string[] { "" }; // nothing to process

    if (ints.Length == 1)
        return new string[] { ints[0].ToString() }; // nothing to process

    Array.Sort<int>(ints); // need to sort these lil' babies
    List<string> values = new List<string>();

    int lastNumber  = ints[0]; // start with the first number
    int firstNumber = ints[0]; // same as above

    for (int i = 1; i < ints.Length; i++)
    {
        int current     = ints[i];
        int difference  = (lastNumber - current ); // compute difference between last number and current number

        if (difference == -1) // the numbers are adjacent
        {
            if (firstNumber == 0) // this is the first of the adjacent numbers
            {
                firstNumber = lastNumber;
            }
            else // we're somehow in the middle or at the end of the adjacent number set
            {
                lastNumber = current;
                continue;
            }
        }
        else
        {
            if (firstNumber > 0 && firstNumber != lastNumber) // get ready to print a set of numbers
            {
                values.Add(string.Format("{0}-{1}", firstNumber, lastNumber));
                firstNumber = 0; // reset
            }
            else // print a single value
            {
                values.Add(string.Format("{0}", lastNumber));
            }
        }

        lastNumber = curren