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())
now=0
T=list(map(int,input().split()))
for i in range(1,N):
    T[i]+=T[i-1]
T=[-1*float('inf'),0]+T+[float('inf')]
V=list(map(int,input().split()))
V=[0]+V+[0]
vt=[float('inf')]*(T[-2]+1)
for t in range(len(vt)):
    now=t
    for k in range(N+2):
        if now<T[k]:
            vt[t]=min(vt[t],V[k]+T[k]-now)
        elif T[k]<=now<=T[k+1]:
            vt[t]=min(vt[t],V[k])
        elif T[k+1]<now:
            vt[t]=min(vt[t],V[k]+now-T[k+1])
ans=0
for i in range(len(vt)-1):
    if abs(vt[i+1]-vt[i])==1:
        ans+=0.5*(vt[i]+vt[i+1])
    elif vt[i+1]==vt[i]:
        for k in range(N+2):
            if T[k]<=i and i+1<=T[k+1]:
                vv=V[k]
                break
        if vv>vt[i]:
            ans+=0.5*(vt[i]*2+0.5)
        else:
            ans+=vt[i]
print(ans)