Untitled

int numberOfContestanstThisYear, numberOfContestantsLastYear
bool isNumber = false;

do {
    Console.WriteLine("Enter the number of contestants of last year >> (number beetween 0 and 30 inclusive)");
    isNumber = int.TryParse(Console.ReadLine(), out numberOfContestsLastYear);
} while (! isNumber || numberOfContestantsLastYear < 0 || numberOfContestantsLastYear > 30);