❔ ✅ is it possible to ''lock'' a var after the first assignment?

/*
Part 1

Santa is trying to deliver presents in a large apartment building, but he can't find the right floor - the directions he got are a little confusing. He starts on the ground floor (floor 0) and then follows the instructions one character at a time.

An opening parenthesis, (, means he should go up one floor, and a closing parenthesis, ), means he should go down one floor.
*/


/*
Part 2

Now, given the same instructions, find the position of the first character that causes him to enter the basement (floor -1). The first character in the instructions has position 1, the second character has position 2, and so on.
*/

using System.Text;

namespace AOC_2015_1
{
    internal class DayOnePartTwo
    {
        static void Main(string[] args)
        {
            string? input;
            Console.WriteLine("Where should I delivery the next gift?");
            Console.SetIn(new StreamReader(Console.OpenStandardInput(), Encoding.UTF8, false, 8192));
            input = Console.ReadLine();
            Console.WriteLine($"Final floor is {GiftDelivery(input)}");
        }

        static short GiftDelivery(string input)
        {
            short floor = 0;
            const char floorUpChar = '(';
            const char floorDownChar = ')';
            int i = 0;
            int x = 0;

            foreach (char c in input)
            {
                if (floor == -1)
                {
                    x = i;
                    Console.WriteLine("is " + x);
                }
                if (c == floorUpChar)
                {
                    floor++;
                    i++;
                }
                else if (c == floorDownChar)
                {
                    floor--;
                    i++;
                }
            }
            return floor;
        }
    }
}