Problem: 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

Solution (in PYTHON3):

Solution:
"""
prd_xxx 様のコードを参考にさせていただきました。感謝です...！
https://beta.atcoder.jp/contests/abc076/submissions/1724160
"""

N = int(input())
T = list(map(int, input().split()))
V = list(map(int, input().split()))

maxv = [0]
# グラフを整える準備
for v, t in zip(V, T):
    # 転換地点では低い方に合わせる
    maxv[-1] = min(maxv[-1], v)
    for ti in range(t * 2):
        maxv.append(v)

T = len(maxv)
# 前からグラフを整える
for i in range(T - 1):
    maxv[i + 1] = min(maxv[i+1], maxv[i] + 0.5)

maxv[-1] = 0
# 後ろからグラフを整える
for i in reversed(range(T-1)):
    maxv[i] = min(maxv[i], maxv[i + 1] + 0.5)

ans = 0
# 台形計算
for i in range(T-1):
    ans += (maxv[i] + maxv[i+1]) * 0.25
print(ans)