C$++$ Using loops to keep a total

Assignment: Imagine you are a teacher and have recently conducted an exam, and you want to know the average grade.

To calculate the average you need to sum up all the grades that were achieved so you can divide that by the amount of students that took the exam. Grades in this exam range from $0$ to $10,$ any other grade is invalid.

Two double variables named sum and grade have been declared.

Write a loop that reads all of the grades as keyboard input $($do not display a prompt$),$ and calculates the sum of all the grades entered. The loop should end when an invalid grade has been entered.