There will be times when you need to create variables that store constant, unchanging values. Adding the const keyword after a variable's access modifier will do just that, but only for built-in C# types. A good candidate for a constant value is our maxItems in the GameBehavior class:
public const int maxItems = 4;
The problem you'll run into with constant variables is that they can only be assigned a value in their declaration, meaning we can't leave maxItems without an initial value:
public readonly int maxItems;
Using the readonly keyword to declare a variable will give us the same unmodifiable value as a constant, while still letting us assign its initial value at any time.