In Python, write function letter2number() that takes as input a letter grade (A, B, C, D, F, possibly with a - or +) and returns the corresponding number grade. The numeric values for A, B, C, D, and F are 4, 3, 2, 1, 0. A + increases the number grade value by 0.3 and a - decreases it by 0.3.

>>> letter2number('A-') 3.7

>>> letter2number('B+') 3.3

>>> letter2number('D') 1.0