In the year 2168, AtCoder Inc., which is much larger than now, is starting a limited express train service called AtCoder Express.

In the plan developed by the president Takahashi, the trains will run as follows:

* A train will run for (t_1 + t_2 + t_3 + ... + t_N) seconds.
* In the first t_1 seconds, a train must run at a speed of at most v_1 m/s (meters per second). Similarly, in the subsequent t_2 seconds, a train must run at a speed of at most v_2 m/s, and so on.



According to the specifications of the trains, the acceleration of a train must be always within ±1m/s^2. Additionally, a train must stop at the beginning and the end of the run.

Find the maximum possible distance that a train can cover in the run.

Constraints

* 1 \leq N \leq 100
* 1 \leq t_i \leq 200
* 1 \leq v_i \leq 100
* All input values are integers.

Input

Input is given from Standard Input in the following format:


N
t_1 t_2 t_3 … t_N
v_1 v_2 v_3 … v_N


Output

Print the maximum possible that a train can cover in the run.
Output is considered correct if its absolute difference from the judge's output is at most 10^{-3}.

Examples

Input

1
100
30


Output

2100.000000000000000


Input

2
60 50
34 38


Output

2632.000000000000000


Input

3
12 14 2
6 2 7


Output

76.000000000000000


Input

1
9
10


Output

20.250000000000000000


Input

10
64 55 27 35 76 119 7 18 49 100
29 19 31 39 27 48 41 87 55 70


Output

20291.000000000000
n = int(input())
time = list(map(int, input().split()))
speed = list(map(int, input().split()))
speed = [0] + speed + [0]
cumtime = [0] * (n+1)
for i in range(n):
    cumtime[i+1] = cumtime[i] + time[i]
dis = 0.0
prev = 0
l = cumtime[n]
tp = 1
for t in range(1, l+1):
    if t > cumtime[tp]:
        tp += 1
    v = speed[tp]
    v = min(v, prev + 1)
    for i in range(tp):
        v = min(v, t - cumtime[i] + speed[i])
    for i in range(tp+1, n+2):
        v = min(v, cumtime[i-1] - t + speed[i])
    if v == prev and v < speed[tp]:
        dis += v + 1/4
    else:
        dis += (v + prev) / 2
    prev = v
print(dis)