A teacher types student marks for a quick check. Write a program that:
Repeatedly asks for a mark (float).
Stops when the user enters -5 (end of input).
Only marks from 50 to 100 inclusive count as valid-ignore anything outside that range (do not count them in the average).
After -5, if there was at least one valid mark, print the average of valid marks. If there were no valid marks, print a short message like No valid marks entered.