private static Dictionary<string, List<string>> OptimizeContacts(List<string> contacts)
{
    var dictionary = new Dictionary<string, List<string>>();

    foreach (var contact in contacts)
    {
        var name = contact.Split(':')[0];
        string namePrefix;
        if (name.Length <= 2)
        {
            namePrefix = name;
        }
        else
        {
            namePrefix = name.Substring(0, 2);
        }
        if (dictionary.ContainsKey(namePrefix))
        {
            dictionary[namePrefix].Add(contact);
        }
        else
        {
            dictionary[namePrefix] = new List<string>() { contact };
        }
    }
    return dictionary;
}