#include<bits/stdc++.h>
using namespace std;
// #include <ext/pb_ds/assoc_container.hpp> // Policy_based_Data_Structure
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;

/*#include <boost/multiprecision/cpp_int.hpp>
using namespace boost::multiprecision;*/

#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")

#define ll long long int
#define ld long double
#define bigint int64_t
#define fst first
#define scd second
#define vll vector<ll>
#define vpll vector<pair<ll,ll>>
#define mp make_pair
#define pb push_back
#define MOD 1000000007
#define flash ios_base::sync_with_stdio(false);cin.tie(NULL);
#define test ll t;read(t);while(t--)
#define all(v) v.begin(),v.end()
#define pll pair<ll,ll>
#define INF (ll)(3e18)
#define vvll vector<vector<ll>>
#define R_0 return 0

#define db(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) { cout << name << " : " << arg1 << '\n'; }
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
	const char* comma = strchr(names + 1, ',');
	cout.write(names, comma - names) << " : " << arg1 << " | "; __f(comma + 1, args...);
}

#define forn(i,n) for(ll (i) = 0 ; (i) < (n) ; ++(i))
#define for1(i,n) for(ll (i) = 1 ; (i) <= (n) ; ++(i))
#define forr(i,n) for(ll (i) = (n)-1 ; (i)>=0 ; --(i))
#define forab(i,a,b,c) for(ll (i) = a ; (i) <= (b) ; (i)+=(c))

template <typename T>
void read(T &x) {
	cin >> x;
}
template <typename T , typename T0>
void read(T &x, T0 &y) {
	cin >> x >> y;
}

template <typename T , typename T0 , typename T1>
void read(T &x, T0 &y, T1 &z) {
	cin >> x >> y >> z;
}

template <typename T , typename T0 , typename T1 , typename T2>
void read(T &x, T0 &y, T1 &z, T2 &w) {
	cin >> x >> y >> z >> w;
}

template <typename T , typename T0>
void read(pair< T , T0 > &p) {
	cin >> p.fst >> p.scd;
}

template <typename T>
void read(vector< T > &oneD, ll n) {
	forn(i, n)
	{
		ll x;
		read(x);
		oneD.push_back(x);
	}
}

template <typename T>
void read(T oneD[] , ll n) {
	for (ll i = 0; i < n; i++) {
		read(oneD[i]);
	}
}

template <typename T>
void write(T &x) {

	cout << x << " ";
}

template <typename T , typename T0>
void write(T &x, T0 &y) {
	cout << x << " " << y << "\n";
}

template <typename T , typename T0 , typename T1>
void write(T &x, T0 &y, T1 &z) {
	cout << x << " " << y << " " << z << "\n";
}

template <typename T , typename T0 , typename T1 , typename T2>
void write(T &x, T0 &y, T1 &z, T2 &w) {
	cout << x << " " << y << " " << z << " " << w << "\n";
}

template <typename T , typename T0>
void write(pair< T , T0 > &p) {
	write(p.fst);
	write(p.scd);
	cout << endl;
}

template <typename T>
void write(vector< T > &oneD, ll n) {
	for (ll i = 0; i < n; i++) {
		cout << oneD[i] << " ";
	}
	cout << endl;
}

template <typename T>
void write(T oneD[] , int n) {
	for (ll i = 0; i < n; i++) {
		write(oneD[i]);
	}
	cout << endl;
}

template <typename T , typename T0>
void write(map< T , T0 > &mpp) {
	for (auto it : mpp) {
		write(it.fst);
		cout << ": ";
		write(it.scd);
		cout << "\n";
	}
	cout << endl;
}

vll seive;
void Seive() {
	const ll maxn = 1e6 + 5;
	seive.resize(maxn);
	forn(i, maxn) seive[i] = i;
	forab(i, 2, maxn, 2)
	seive[i] = 2;
	seive[1] = -1;
	seive[0] = -1;
	for (ll i = 3; i <= maxn; i += 2) if (i == seive[i]) for (ll j = i * i ; j < maxn ; j += i) if (seive[j] == j) seive[j] = i;
}

ll extended_GCD(ll a , ll b , ll &x , ll &y) {
	if (a == 0) {
		x = 0;
		y = 1;
		return b;
	}
	ll x1 , y1;
	ll gcd = extended_GCD(b % a , a , x1 , y1);
	x = y1 - (b / a) * x1;
	y = x1;
	return gcd;
}

ll modinv(ll a , ll mod = MOD) {
	ll x , y;
	extended_GCD(a , mod , x , y);
	if (x < 0) x += mod;
	return x;
}

//Chinese Remainder Theoram
ll find_x_CRT(ll num[], ll rem[], ll n)
{
	ll pro = 1;
	forn(i, n)
	pro *= num[i];
	ll res = 0;
	forn(i, n)
	{
		ll pp = pro / num[i];
		res += (rem[i] * modinv(pp, num[i]) * pp);
	}
	res %= pro;
	return res;
}

ll power(ll a, ll b, ll m = MOD) {
	a %= m;
	ll res = 1;
	while (b > 0) {
		if (b & 1)
			res = res * a % m;
		a = a * a % m;
		b >>= 1;
	}
	return res;
}

ll powe(ll a, ll b, ll m = MOD) {
	ll res = 1;
	while (b > 0) {
		if (b & 1)
			res = res * a;
		a = a * a;
		b >>= 1;
	}
	return res;
}

ll gcd(ll a, ll b)
{
	if (a == 0)
		return b;
	return gcd(b % a, a);
}

string getString(char x)
{
	string s(1, x);
	return s;
}

void update(vll &BIT, ll index, ll inc)
{
	while (index <= (ll)(BIT.size()))
	{
		BIT[index] += inc;
		index += (index & (-index));
	}
}

ll query(vll &BIT, ll index)
{
	ll sum = 0;
	while (index > 0)
	{
		sum += BIT[index];
		index -= (index & (-index));
	}
	return sum;
}

// typedef tree<ll, null_type, less<ll>, rb_tree_tag, tree_order_statistics_node_update> index_set;
// typedef tree<ll, null_type, less_equal<ll>, rb_tree_tag, tree_order_statistics_node_update> indexed_multiset;
void make_set(ll v, vll &parent, vll &siz)
{
	parent[v] = v;
	siz[v] = 1;
}

ll find_set(ll x, vll &parent)
{
	if (x == parent[x])
		return x;
	else
	{
		return parent[x] = find_set(parent[x], parent);
	}
}

void union_set(ll a, ll b, vll &siz, vll &parent)
{
	a = find_set(a, parent);
	b = find_set(b, parent);
	if (a != b)
	{
		if (siz[a] < siz[b])
			swap(a, b);
		parent[b] = a;
		siz[a] += siz[b];
		siz[b] = 0;
	}
}

void precompute_NCR(vll &fact)
{
	const ll N = 1e6 + 5;
	fact[0] = 1;
	forab(i, 1, (N - 1), 1)
	{
		ll y = (i % MOD * fact[i - 1] % MOD) % MOD;
		fact[i] = y;
	}
}

ll NCR(vll &fact, ll N, ll R, ll mod = MOD)
{
	ll n = fact[N];
	ll r = fact[R];
	ll n_r = fact[N - R];
	ll y = (r % mod * n_r % mod) % mod;
	ll z = modinv(y, mod);
	ll ans = (n % mod * z % mod) % mod;
	return ans;
}

void shuru_karte_hai_bina_kise_bakchodi_ke()
{
	flash;/*
#ifndef ONLINE_JUDGE
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);
#endif*/
}

const ll siz = 10005;
ll chainNo = 0, chainHead[siz], chainPos[siz], chainInd[siz], chainSize[siz];
vvll ad(siz);
ll costs[siz] = {};
bool vis[siz];
ll subtree[siz] = {};

ll dfs(ll src)
{
	vis[src] = true;
	ll ans = 1;
	for (auto x : ad[src])
	{
		if (!vis[x])
		{
			ans += dfs(x);
		}
	}
	subtree[src] = ans;
	return ans;
}

void hld(ll cur)
{
	if (chainHead[chainNo] == -1)
	{
		chainHead[chainNo] = cur;
	}
	chainInd[cur] = chainNo;
	chainPos[cur] = chainSize[chainNo];
	chainSize[chainNo]++;
	ll ma = 0, ind = 0;
	for (auto x : ad[cur])
	{
		if (subtree[x] > ma)
		{
			ma = subtree[x];
			ind = x;
		}
	}
	hld(ind);
	for (auto x : ad[cur])
	{
		if (x != ind)
		{
			chainNo++;
			hld(x);
		}
	}
	return;
}

const ll N = 3000;
ld a[N] = {};
ld dp[N][N] = {};

ld solve(ll i, ll h, ll n)
{
	if (i == n)
	{
		ll t = n - h;
		if (h > t)
			return 1.0;
		return 0.0;
	}
	if (dp[i][h] != -1.0)
		return dp[i][h];
	ld q1 = a[i] * solve(i + 1, (h + 1), n);
	ld q2 = (1.0 - a[i]) * (solve(i + 1, h, n));
	dp[i][h] = (q1 + q2);
	return (q1 + q2);
}

int main()
{
	shuru_karte_hai_bina_kise_bakchodi_ke();
	ll n;
	read(n);
	read(a, n);
	forn(i, n + 1)
	{
		forn(j, n + 1)
		dp[i][j] = -1.0;
	}
	ld ans = solve(0, 0, n);
	cout << fixed << setprecision(11) << ans;
	return 0;
}

Measured memory_bytes: 141440