t, s, d = map(int, input().split())

time = d / s

if t >= 18 or t < 6:
    night_time = time
else:
    if (t + time) >= 18 or (t + time) < 6:
        night_time = time - (18 - t)
    else:
        night_time = 0

print("{:.10f}".format(night_time))