The table below shows the conversion from letter grades to grade points at a particular academic

institution.

In this exercise you will compute the grade point average of an arbitrary number of letter grades

entered by the user. The user will enter a blank line to indicate that all of the grades have been

provided.

For example, if the user enters A$,$ followed by C$+,$ followed by B$,$ followed by a blank

line then your program should report a grade point average of $3\mathrm{.}1.$

Your program needs to do error checking: Each value entered by the user must always be a valid

letter grade or a blank line.