Write a program that translates a letter grade into a number grade. Letter grades are A B C D F, possibly followed by + or . Their numeric values are...

Write a program that translates a letter grade into a number grade. Letter grades are A B C D F, possibly followed by + or −. Their numeric values are 4, 3, 2, 1, and 0. There is no F+ or F−. A + increases the numeric value by 0.3, a − decreases it by 0.3. However, an A+ has the value 4.0. All other inputs have value −1.