#include <bits/stdc++.h>
using namespace std;

#include  <ext/pb_ds/assoc_container.hpp>
#include  <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

template<class T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define pb push_back
#define x first
#define y second
#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (a) - 1; i >= (b); i--)

typedef long long ll;
typedef long double db;
//typedef long double LD;
typedef pair<int, int> pii;
typedef pair<db, db> pdd;
typedef pair<ll, ll> pll;


int solve()
{
	int n;
	if (!(cin >> n))
		return 1;
	
	vector<db> p(n);
	FOR (i, 0, n)
		cin >> p[i];
	
	if (n <= 2)
	{
		cout << "0\n";
		return 0;
	}
	
	vector<int> masks;
	FOR (i, 1, 1 << n)
	{
		masks.pb(i);
	}
	
	sort(all(masks), [](int l, int r)
	{
		return popcount((unsigned int)l) < popcount((unsigned int)r);
	});
	
	vector<db> dp(1 << n, -1);
	for (int mask: masks)
	{
		int pp = popcount((unsigned int)mask);
		if (pp < 3)
			continue;
		
		vector<int> in;
		FOR (j, 0, n)
		{
			if (mask & 1 << j)
				in.pb(j);
		}
		
		if (sz(in) == 3)
		{
			db pr = 0;
			FOR (i, 0, 1 << 3)
			{
				db cr = 1;
				FOR (j, 0, 3)
				{
					if (i & 1 << j)
						cr *= p[in[j]];
					else
						cr *= (1 - p[in[j]]);
				}
				if (i != 0 && i != (1 << 3) - 1)
					pr += cr;
			}
			dp[mask] = 1. / pr;
			continue;
		}
		
		db p1 = 1, p2 = 1;
		for (int j: in)
		{
			p1 *= p[j];
			p2 *= (1 - p[j]);
		}
		db sm = 0;
		db smpr = 0;
		for (int j: in)
		{
			db cp1 = p1 / p[j];
			db cp2 = p2 / (1 - p[j]);
			cp1 *= (1 - p[j]);
			cp2 *= p[j];
			db pr = cp1 + cp2;
			assert(pr <= 1. + 1e-7);
			sm += pr;
			smpr += pr * (dp[mask ^ (1 << j)] + 1);
		}
		assert(sm <= 1. + 1e-7);
		dp[mask] = (smpr + 1 - sm) / sm;
	}
	cout << fixed << setprecision(7) << dp.back() << '\n';
	
	return 0;
}

int32_t main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	int TET = 1e9;
	//cin >> TET;
	for (int i = 1; i <= TET; i++)
	{
		if (solve())
		{
			break;
		}
		#ifdef ONPC
			cerr << "_____________________________\n";
		#endif
	}
	#ifdef ONPC
		cerr << "\nfinished in " << clock() * 1.0 / CLOCKS_PER_SEC << " sec\n";
	#endif
	return 0;
}