#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define ALL(x) ::std::begin(x), ::std::end(x)
using namespace std;

auto solve(int n, const vector<int> & a) {
    vector<int> pow_3(n + 1);
    pow_3[0] = 1;
    REP (i, n) {
        pow_3[i + 1] = pow_3[i] * 3;
    }
    vector<array<double, 2> > dp(pow_3[n]);
    REP_R (state, pow_3[n]) {
        int available = 0;
        int score = 0;
        {  // unpack
            int s = state;
            REP (i, n) {
                if (s % 3 == 0) {
                    available |= 1 << i;
                } else if (s % 3 == 1) {
                    score += a[i];
                } else {
                    score -= a[i];
                }
                s /= 3;
            }
        }
        if (not available) {
            dp[state][false] = score > 0;
            dp[state][true] = score <= 0;
        } else {
            REP (turn, 2) {
                REP (i, n) if (available & (1 << i)) {
                    double prob = 1;
                    REP (j, n) if (available & (1 << j)) {
                        double p = 1.0 / a[i];
                        double q = 1.0 / a[j];
                        if (a[i] == 1) {
                            double x = 1 - dp[state + (turn + 1) * pow_3[i]][not turn];
                            prob = min(prob, x);
                        } else if (a[j] == 1) {
                            double x = p * (1 - dp[state + (turn + 1) * pow_3[i]][not turn]);
                            double y = (1 - p) * dp[state + (not turn + 1) * pow_3[j]][turn];
                            prob = min(prob, x + y);
                        } else {
                            double x = p * (1 - dp[state + (turn + 1) * pow_3[i]][not turn]);
                            double y = (1 - p) * q * dp[state + (not turn + 1) * pow_3[j]][turn];
                            double r = (1 - p) * (1 - q);
                            prob = min(prob, (x + y) / (1 - r));
                        }
                    }
                    dp[state][turn] = max(dp[state][turn], prob);
                }
            }
        }
    }
    return dp[0][false];
}

int main() {
    int n; cin >> n;
    vector<int> a(n);
    REP (i, n) {
        cin >> a[i];
    }
    auto ans = solve(n, a);
    cout << setprecision(18) << ans << endl;
    return 0;
}