private static Dictionary<string, List<string>> OptimizeContacts(List<string> contacts)
{
    var dictionary = new Dictionary<string, List<string>>();
    foreach(var el in contacts)
    {
        var name = el.Split(':')[0];
        var ind = name.Length >= 2? name.Substring(0, 2) : name.Substring(0, 1);
        if(!dictionary.ContainsKey(ind))
            dictionary[ind] = new List<string>();
        dictionary[ind].Add(el);
    }
    return dictionary;
}