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())
T = list(map(int, input().split()))
V = list(map(int, input().split()))
M = 2
dt = 1 / M
R = sum(T)

maxSpeeds = [10**18] * (R * M + 1)
now = 0
for t, v in zip(T, V):
    for i in range(t * M + 1):
        maxSpeeds[now + i] = min(maxSpeeds[now + i], v)
    now += t * M

maxSpeeds[0] = 0
maxSpeeds[-1] = 0
for t in range(1, R * M + 1):
    maxSpeeds[t] = min(maxSpeeds[t - 1] + dt, maxSpeeds[t])
    maxSpeeds[-(t + 1)] = min(maxSpeeds[-t] + dt, maxSpeeds[-(t + 1)])

ans = 0
for l, r in zip(maxSpeeds, maxSpeeds[1:]):
    ans += (l + r) / M / 2
print(ans)