When I want to use (when keyword) with switch and case it gives me a lot of errors

I have a homework assignment that requires me to use switch and case statements. For each case, if necessary, I should use 'when'. When I write all the code, it gives me a lot of errors. Here's a sample of the code. If you can help me, thank you

int mark;mark = Convert.ToInt32(Console.ReadLine());

        switch(mark)        {            case  int Excellent when  excellent >= 90:                Console.WriteLine("excellent");                break;            case int Verygood when verygood >=80 && verygood<90:                Console.WriteLine("very good");                break;           case int good when good>=70 && good<80:                Console.WriteLine("good");                break;           case int Average when Average>=60 && Average<70:                Console.WriteLine("Average");                break;            case int JustPass when JustPass>=50 && JustPass<60:                Console.WriteLine("Just Pass");                break;            default:                Console.WriteLine("fail");                break;        }

When a user enters their grade, I want the program to output whether it is 'good', 'very good', or 'bad'.