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
INF = 10**18

N = int(input())
T = list(map(int, input().split()))
V = list(map(int, input().split()))
V = [0] + V + [0]
R = [(0, 0)]*(N+2)
for i in range(1,N+1):
    R[i] = (R[i-1][1], R[i-1][1] + T[i-1])
R[N+1] = (R[N][1], R[N][1])
def velocity(i, x):
    if x < R[i][0]:
        return V[i]+R[i][0]-x
    elif R[i][0] <= x <= R[i][1]:
        return V[i]
    else:
        return V[i]+x-R[i][1]
ans = 0
dp = [0]*(R[N+1][1]*2+1)
for x in range(1, R[N+1][1]*2+1):
    mi = INF
    for i in range(N+2):
        mi = min(velocity(i, x/2), mi)
    dp[x] = mi
    ans += 0.25*(dp[x-1]+mi)
print(ans)
