a = int (input())
b = int (input())
s = 0
n = 0
for i in range (a, b + 1):
    if a % 3 != 0:
        a +=1
    else:
        break
if s % 3 == 0
	n +=1
for j in range (i, b + 1, 3):
    s +=j
print (s / n)
