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Handle possible exceptions across your code. [Handle Exceptions][1]

Handle parsing values directly to avoid exceptions (use TryParse instead of Parse).

int VolbaObrazce;
bool validInput= int.TryParse(Console.ReadLine(),out VolbaObrazce);
//here you shall have value in VolbaObrazce if you only provide valid int.

you have also multiple methods that handles only arithmetic operation which will be very hard to maintain. [1]: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/try-catch

Handle possible exceptions across your code. Handle Exceptions

Handle parsing values directly to avoid exceptions (use TryParse instead of Parse).

int VolbaObrazce;
bool validInput= int.TryParse(Console.ReadLine(),out VolbaObrazce);
//here you shall have value in VolbaObrazce if you only provide valid int.

you have also multiple methods that handles only arithmetic operation which will be very hard to maintain.