第一遍没搞明白怎么输出的，没用字典的笨办法

a = []
while True:
    num = input()
    if num=='False':
        break
    else:
        a.append(num)
b = 0
c = 0
for i in range(0, len(a),2):
    if a[i] == 'A':
        b += int(a[i+1])*4.0
        c += int(a[i+1])
    elif a[i] == 'B':
        b += int(a[i+1])*3.0
        c += int(a[i+1])
    elif a[i] == 'C':
        b += int(a[i+1])*2.0
        c += int(a[i+1])
    elif a[i] == 'D':
        b += int(a[i+1])*1.0
        c += int(a[i+1])
    elif a[i] == 'F':
        b += int(a[i+1])*0
        c += int(a[i+1])
    else:
        break
num = b/c
        
print('%.2f'%num)