#include <cstdio>
#include <algorithm>
using namespace std;
namespace Solver {
	int m;
	double a[23], b[23];
	double solve() {
		// p[i] from 0 to 1
		// E    from a[i] to b[i]
		// x = 增加 1 需要多少金额.
		// y = 已经分配了多少.
		double x = 1 / (a[0] - b[0]), y = 0.0;
		for (int i = 1; i < m; i++) {
			double d = a[i - 1] - a[i];
			if (y + d * x > 1) {
				return a[i - 1] - (1 - y) / x;
			}
			if (a[i] <= b[i]) {
				return a[i];
			}
			y += d * x;
			x += 1 / (a[i] - b[i]);
		}
		return a[m - 1] - (1 - y) / x;
	}
}
int a[23];
double f[1 << 23];
int main() {
//	freopen("in.txt", "r", stdin);
	int n;
	scanf("%d", &n);
	int len = 0;
	for (int i = 0; i < n; i++) {
		scanf("%d", &a[len]);
		if (a[len] > 0) {
			len++;
		}
	}
	n = len;
	sort(a, a + n);
	reverse(a, a + n);
	f[0] = 0.0;
	for (int S = 1; S < (1 << n); S++) {
		Solver::m = 0;
		for (int i = 0; i < n; i++) {
			if (((S >> i) & 1) == 1) {
				Solver::a[Solver::m] = a[i];
				Solver::b[Solver::m] = f[S ^ (1 << i)];
				Solver::m++;
			}
		}
		f[S] = Solver::solve();
	}
	printf("%.10lf", f[(1 << n) - 1]);
	return 0;
}