def main():

    S = list(input())
    checklist = [0 for i in range(26)]
    for i in S:
        checklist[ord(i) - ord("a")] += 1
    ans = True

    for i in checklist:
        if i > 1:
            ans = False

    if not ans:
        print(-1)
        return 0
    else:
        ans = False
        for i in range(len(checklist)):
            if checklist[i] == 0:
                for i in S:
                    print(i, end="")
                print(chr(i + ord("a")), end = "")
                print()
                ans = True
                break

        if not ans:
            new_S = []
            for i in S:
                new_S.append(ord(i))

            for i in range(len(new_S) - 1):
                if new_S[i] > new_S[i + 1]:
                    for j in range(i - 1):
                        print(S[j], end = "")
                    print(chr(new_S[i - 1] + 1), end = "")
                    print()
                    ans = True
                    break

            if not ans:
                print(-1)
#main()

def ABC069D():

    H, W = map(int, input().split())
    N = int(input())
    a = list(map(int, input().split()))
    ans = []
    for i in range(N):
        for j in range(a[i]):
            ans.append(i + 1)
    printlist = [[0 for i in range(W)] for j in range(H)]
    k = 0
    for i in range(H):
        for j in range(W):
            printlist[i][j] = ans[k]
            k += 1
        if i % 2 != 0:
            printlist[i] = printlist[i][::-1]

    for i in range(H):
        for j in range(W):
            print(printlist[i][j], end = "")
            if j != W:
                print(" ", end = "")
        print()
    print()
#ABC069D()

import math
def cc(n, r):
    return math.factorial(n) // (math.factorial(n - r) * math.factorial(r))

def ABC057D():

    N, A, B = map(int, input().split())
    v = list(map(int, input().split()))
    v = sorted(v)[::-1]

    anslist = []
    for i in range(A, B + 1):

        anslist.append([sum(v[0:i]) / i, i, v[i - 1]])
    anslist.sort()
    anslist = anslist[::-1]
    ans = anslist[0][0]
    key = anslist[0][2]
    long = anslist[0][1]
    print(ans)
    for i in range(1, len(anslist)):
        if anslist[i][0] == anslist[i - 1][0]:
            ans = anslist[i][0]
            key = anslist[i][2]
            long = anslist[i][1]
        else:
            break
    #print(anslist)
    #print(ans, key, long)
    cnt = 0
    for i in v:
        if i == key:
            cnt += 1

    c = 0
    for i in v[:long]:
        #print(i)
        if i == key:
            c += 1
    #print(c, long)
    ans = 0
    if c == long:
        for i in range(A,  min(B + 1, cnt + 1)):
            ans += cc(cnt, i)
    else:
        ans = cc(cnt, c)
    #print(anslist)
    #print(cnt, c, long)
    print(ans)
#ABC057D()

def ABC104D():
    S = list(input())
    A_cnt = 0
    B_cnt = 0
    C_cnt = 0
    Q_cnt = 0
    ans = 0
    for i in S:
        if i == "A":
            A_cnt += 1
        elif i == "B":
            B_cnt += 1
        elif i == "C":
            C_cnt += 1
        else:
            Q_cnt += 1
    for i in range(Q_cnt + 1):
        for j in range(Q_cnt - i + 1):
            k = Q_cnt - i - j
            if A_cnt + i > 0 and B_cnt + j > 0 and C_cnt + k > 0:
                f_i = math.factorial(i)
                f_j = math.factorial(j)
                f_k = math.factorial(k)
                if f_i == 0:
                    f_i = 1
                if f_j == 0:
                    f_j = 1
                if f_k == 0:
                    f_k = 1
                ans += (A_cnt + i) * (B_cnt + j) * (C_cnt + k) * math.factorial(Q_cnt) // f_i // f_j // f_k
                print(ans, i, j, k, A_cnt + i, B_cnt + j, C_cnt + k)
                print(math.factorial(Q_cnt), f_i, f_j, f_k)
    print(ans)
#ABC104D()

def ARC073D():
    N, W = map(int, input().split())
    wv = [list(map(int, input().split())) for i in range(N)]

def ARC075():
    N, A, B = map(int, input().split())
    h = [int(input()) for i in range(N)]
    def enogth(T):
        import copy
        this_h = copy.copy(h)
        cnt = 0
        for i in range(N):
            this_h[i] -= B
            key = 0
            if this_h[i] > 0:
                key = this_h[i] / (A - B)
                if int(key) != key:
                    key += 1
                cnt += int(key)
        if cnt <= T:
            return True
        else:
            return False
    def sarch(i: object) -> object:
        print(i)
        if enogth(i):
            if i // 2 == 1:
                return i
            sarch(0 + i // 2)
        else:
            sarch(i + i // 2)
    print(sarch(10 ** 9))
#ARC075()

def ARC082D():
    N = int(input())
    p = list(map(int, input().split()))
    cnt = 0
    if p[N - 1] == N:
        #print(p[N - 1], N)
        p[-2], p[-1] = p[-1], p[-2]
        cnt += 1

    for i in range(N - 1):
        if p[i] == i + 1:
            p[i], p[i + 1] = p[i + 1], p[i]
            cnt += 1
    print(cnt)
#ARC082D()


from queue import Queue
def ABC070D():
    N = int(input())
    a, b, c =map(int, input().split())
    Q, K = map(int, input().split())
    xy = [list(map(int, input().split())) for i in range(Q)]
    x = [[] for i in range(N)]
    for i, j in xy:
        x[i - 1].append(j - 1)
        x[j - 1].append(i - 1)

    q = Queue()
    visited = [0 for i in range(N)]


def ABC076D():
    N = int(input())
    t = list(map(int, input().split()))
    v = list(map(int, input().split()))
    speed_lim = [0]
    speed_list = [0 for i in range(sum(t) + 1)]
    for i in range(N):
        for j in range(t[i]):
            speed_lim.append(v[i])
    print(speed_lim)
    for i in range(1, sum(t) + 1):
        speed_list[i] = speed_list[i - 1] + 1
        if speed_list[i] > speed_lim[i]:
            speed_list[i] = speed_lim[i]
    print(speed_list)
    speed_list[-1] = 0
    for i in range(sum(t) - 1, -1, -1):
        speed_list[i] = min(speed_list[i + 1] + 1, speed_list[i])
        if speed_list[i] > speed_lim[i + 1]:
            speed_list[i] = speed_lim[i + 1]
    print(speed_list)
    ans = 0
    for i in speed_list:
        ans += i

    print(ans)
#ABC076D()

def ABC076D():
    N = int(input())
    t = list(map(int, input().split()))
    a = []
    for i in t:
        a.append(i * 2)
    t = a
    v = list(map(int, input().split()))
    #print(t)
    speed_lim = [0]
    speed_list = [0 for i in range(sum(t) + 1)]
    for i in range(N):
        for j in range(t[i]):
            speed_lim.append(v[i])
    #print(speed_lim, len(speed_list), len(speed_lim))
    for i in range(1, sum(t) + 1):
        speed_list[i] = speed_list[i - 1] + 0.5
        if speed_list[i] > speed_lim[i]:
            speed_list[i] = speed_lim[i]
    #print(speed_list)
    speed_list[-1] = 0
    for i in range(sum(t) - 1, -1, -1):
        speed_list[i] = min(speed_list[i + 1] + 0.5, speed_list[i])
        if speed_list[i] > speed_lim[i + 1]:
            speed_list[i] = speed_lim[i + 1]
    #print(speed_list)
    ans = 0
    for i in speed_list:
        ans += i

    print(ans / 2)
ABC076D()

Measured memory_bytes: 4780