Variables reset to zero when I call a method
using System;

namespace Assignment7
{
    class Plane
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Welcome to the Airline Reservation System.");
            Console.WriteLine("Where would you like to sit?n");
            Console.WriteLine("Enter 1 for First Class.");
            Console.WriteLine("Enter 2 for Economy.");
            CheckIn();
        }

        public static void CheckIn()
        {
            int choice = Convert.ToInt32(Console.ReadLine());
            int firstClass = 0;
            int economy = 0;

            if (choice == 1)
            {
                do
                {
                    Console.WriteLine("You have chosen a First Class seat.");
                    firstClass++;
                    CheckIn();
                } while (firstClass < 5);
            }
            else if (choice == 2)
            {
                do
                {
                    Console.WriteLine("You have chosen an Economy seat.");
                    economy++;
                    CheckIn();
                } while (economy < 5);
            }
            else
            {
                Console.WriteLine("That does not compute.");
                CheckIn();
            }
        }
    }
}

int firstClass = 0;

static int firstClass = 0;

using System;

namespace Assignment7
{
    class Plane
    {
        public static void Main(string[] args)
        {
            Console.WriteLine("Welcome to the Airline Reservation System.");
            Console.WriteLine("Where would you like to sit?n");
            Console.WriteLine("Enter 1 for First Class.");
            Console.WriteLine("Enter 2 for Economy.");
            CheckIn(0, 0);
        }

        public static void CheckIn(int firstClassSeatsTaken, int economySeatsTaken)
        {
            int choice = Convert.ToInt32(Console.ReadLine());

            if (choice == 1)
            {
                do
                {
                    Console.WriteLine("You have chosen a First Class seat.");
                    firstClass++;
                    CheckIn(firstClassSeatsTaken, economySeatsTaken);
                } while (firstClass < 5);
            }
            else if (choice == 2)
            {
                do
                {
                    Console.WriteLine("You have chosen an Economy seat.");
                    economy++;
                    CheckIn(firstClassSeatsTaken, economySeatsTaken);
                } while (economy < 5);
            }
            else
            {
                Console.WriteLine("That does not compute.");
                CheckIn(firstClassSeatsTaken, economySeatsTaken);
            }
        }
    }
}