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())) 

operations = T[:]
v_limits = V[:]

v = 0
limits_x = [0]
for i in range(N):
    for _ in range(operations[i]*2):
        v += 0.5
        limit = min(v, v_limits[i])
        limits_x.append(limit)
        v = limit
        
        
v = 0
limits_y = [0]
for i in reversed(range(N)):
    for _ in range(operations[i]*2):
        v += 0.5
        limit = min(v, v_limits[i])
        limits_y.append(limit)
        v = limit
limits_y = list(reversed(limits_y))
#print(limits_x)
#print(limits_y)

limits_z = [min(x,y) for x, y in zip(limits_x, limits_y)]
#print(limits_z)

ans = 0
for _z, z in zip(limits_z[:-1],limits_z[1:]):
    ans += (_z+z)*0.5/2
print(ans)

