求大神解答第四题！！

Question

 

Answer 1

def a(L):
    for i in L:
        if i not in ['A', 'B', 'C', 'D', 'F']:
            return -1
        else:
            result = L.count('A') * 4 + L.count('B') * 3+ L.count('C') * 2+ L.count('D') * 1
    return format(result/len(L), '.2f')

L = ['A', 'B', 'C', 'D', 'F']
a(L)