At the start of the program, the user enters the initial amount of gold. Then he is offered to buy a certain number of crystals at the price of N. The user enter a number and his gold is converted into crystals. The rest of the gold and crystals are displayed on the screen.

Crystal Shop

namespace MyCode
{
    internal class Program
    {
        static void Main(string[] args)
        {
            int gold;
            int crystals;
            int crystalPrice = 5;

            Console.WriteLine("Enter your amount of gold");
            gold = int.Parse(Console.ReadLine());

            Console.WriteLine($"Price for each crystal is {crystalPrice}. How much do You want to buy?");
            crystals = int.Parse(Console.ReadLine());
            
            gold -= crystalPrice * crystals;
            
            Console.WriteLine($"Curreently you have {gold} of gold and {crystals} of crystals");
        }
    }
}