The type or namespace name 'Calculator' could not be found (are you missing a using directive or an assembly reference?)

The type or namespace name 'Calculator' could not be found (are you missing a using directive or an assembly reference?)

Process completed with exit code 1.

Prefer 'static readonly' fields over constant array arguments if the called method is called repeatedly and is not mutating the passed array (https://learn.microsoft.com/dotnet/fundamentals/code-analysis/quality-rules/ca1861)