#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = l; i <= r; ++i)
#define per(i, r, l) for (int i = r; i >= l; --i)
using namespace std;
typedef long long ll;
const int N = 17;
const int M = 1 << N;

struct matrix {
    double a[2][2];
    matrix operator*(const matrix &rhs) const {
        matrix ret;
        rep(i, 0, 1) {
            rep(j, 0, 1) {
                ret.a[i][j] = 0;
                rep(k, 0, 1) { ret.a[i][j] += a[i][k] * rhs.a[k][j]; }
            }
        }
        return ret;
    }
    matrix operator^(const int &k) const {
        matrix m = *this;
        matrix ret;
        rep(i, 0, 1) rep(j, 0, 1) ret.a[i][j] = (i == j);
        int y = k;
        while (y) {
            if (y & 1) ret = ret * m;
            m = m * m;
            y >>= 1;
        }
        return ret;
    }
};

double f(int a, int b, double p) {
    matrix m;
    m.a[0][0] = 0, m.a[0][1] = 1;
    m.a[1][0] = 1 - 1 / p, m.a[1][1] = 1 / p;
    matrix mn = m ^ (a + b);
    double x = 1 / mn.a[0][1];
    matrix ma = m ^ a;
    double ret = ma.a[0][1] * x;
    // printf("f(%d,%d,%lf)=%lf\n", a, b, p, ret);
    return ret;
}

int n;
double p[N];
int s[N];
double dp[M];

int main() {
    scanf("%d", &n);
    rep(i, 0, n - 1) { scanf("%lf", &p[i]); }
    s[0] = 0;
    rep(i, 1, n) { scanf("%d", &s[i]); }
    dp[0] = 1;
    rep(S, 1, (1 << n) - 1) {
        int t = __builtin_popcount(S);
        dp[S] = 0;
        rep(i, 0, n - 1) {
            if (S & (1 << i)) {
                int S0 = S - (1 << i);
                dp[S] = max(dp[S],
                            dp[S0] * f(s[t] + s[t - 1], s[t] - s[t - 1], p[i]) +
                                (1 - dp[S0]) *
                                    f(s[t] - s[t - 1], s[t] + s[t - 1], p[i]));
            }
        }
        // printf("%d %lf\n", S, dp[S]);
    }
    printf("%.8lf\n", dp[(1 << n) - 1]);
    return 0;
}