public static int sockMerchant(int n, List<int> ar)
    {
        var orderArray = ar.Order().ToList();
        
        var sets = orderArray.GroupBy(n => n).ToList();
        
        int countPairs = 0;
        
        sets.ForEach(s => countPairs += s.ToList().Count() / 2);
        
        return countPairs;
    }