Write a program that keeps asking the user to enter students' grades until he enters -1, then your program should display how many entered grades and the average of these grades. Your program should only accept grades between 0 and 100, any other value is not counted. Here is an example of the expected output: Please enter grade 1 (or - 1 to exit): 85.5 Please enter grade 2 (or - 1 to exit): 77.5 Please enter grade 3 (or - 1 to exit): 105 Incorrect input. Please enter grade 3 (or - 1 to exit): 95 Please enter grade 4 (or - l to exit): - 1 You entered 3 valid grades. The average is 86