n = int(input())
A = list(map(int, input().split()))
R = list(map(int, input().split()))
ans = 0
for i in range(n):
    dp = [1]
    for j in range(n):
        if i == j:
            l = len(dp)
            ans += R[j] * A[i] * dp[0] / 2
            dp2 = [0] * l
            for k in range(l):
                dp2[k] += dp[k] / 2
                if k != 0:
                    dp2[k - 1] += dp[k] / 2
            dp = dp2
        elif j < i:
            l = len(dp)
            if A[j] <= A[i]:
                dp2 = [0] * (l + 1)
                for k in range(l):
                    dp2[k] += dp[k] / 2
                    dp2[k + 1] += dp[k] / 2
            else:
                dp2 = [0] * l
                for k in range(l):
                    dp2[k] += dp[k] / 2
                    dp2[max(0, k - 1)] += dp[k] / 2
            dp = dp2
        else:
            l = len(dp)
            if A[j] < A[i]:
                dp2 = [0] * (l + 1)
                for k in range(l):
                    dp2[k] += dp[k] / 2
                    dp2[k + 1] += dp[k] / 2
            else:
                ans += R[j] * A[i] * dp[0] / 2
                dp2 = [0] * l
                for k in range(l):
                    dp2[k] += dp[k] / 2
                    if k != 0:
                        dp2[k - 1] += dp[k] / 2
            dp = dp2
        
print(ans)