import sys
a = []
data_flag = 0
score = 0
timer = 0

score_tycle = {"A" :4.0,"B": 3.0,"C": 2.0,"D": 1.0,"F": 0}

for line in sys.stdin:
    a +=line.split()

for x in a:
    if x == "False":
        break
    else:
        if data_flag == 0:
            if x in score_tycle:
                tem = float(score_tycle[x])
                data_flag = 1
        else:
            try:
                data_flag = 0
                tem  = tem*float(x)
                timer =float(x) + timer
                score  = tem+score
            except ValueError:
                continue
print("%0.2f"%(score/timer))