function that, given the number of rows N and a list of reserved seats as string S returns the maximum of four-person families that can be seated in the remaining unreserved seats c++ code example

Example: function that, given the number of rows N and a list of reserved seats as string S returns the maximum of four-person families that can be seated in the remaining unreserved seats c++

public int solution(int N, string S)
        {
            int number=1;
            int counter = 0;
            List listReserved = new List();
            listReserved = S.Split(' ').ToList();
            while(number<=N)             {                 if (listReserved.Where(x=> x.Contains(number + "B") || x.Contains(number + "C")|| x.Contains(number + "D")|| x.Contains(number + "E")).FirstOrDefault()==null)
                {
                    listReserved.AddRange(new List{number + "B", number + "C",number + "D" , number + "E"});
                    counter++;
                }

                if (listReserved.Where(x=> x.Contains(number + "F") || x.Contains(number + "G")|| x.Contains(number + "H")|| x.Contains(number + "J")).FirstOrDefault()==null)
                {
                    listReserved.AddRange(new List{number + "F", number + "G",number + "H" , number + "J"});
                    counter++;
                }
                
                if (listReserved.Where(x=> x.Contains(number + "D") || x.Contains(number + "E")|| x.Contains(number + "F")|| x.Contains(number + "G")).FirstOrDefault()==null)
                {
                    listReserved.AddRange(new List{number + "D", number + "E",number + "F" , number + "G"});
                    counter++;
                }
                
                number++;
            }
           
            return counter;
        }