need help in c# plz

Write an application named DailyTemps that continuously prompts a user for a series of daily high temperatures until the user enters a sentinel value of 999 . Valid temperatures range from 20 through 130 Fahrenheit. When the user enters a valid temperature, add it to a total; when the user enters an invalid temperature, display the error message: Valid temperatures range from 20 to 130 . Please reenter temperatur Before the program ends, display the number of temperatures entered and the average temperature