#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"

#define PB push_back
#define PF push_front
#define LB lower_bound
#define UB upper_bound
#define fr(x) freopen(x,"r",stdin)
#define fw(x) freopen(x,"w",stdout)
#define iout(x) printf("%d\n",x)
#define lout(x) printf("%lld\n",x)
#define REP(x,l,u) for(ll x = l;x<u;x++)
#define RREP(x,l,u) for(ll x = l;x>=u;x--)
#define complete_unique(a) a.erase(unique(a.begin(),a.end()),a.end())
#define mst(x,a) memset(x,a,sizeof(x))
#define all(a) a.begin(),a.end()
#define PII pair<int,int>
#define PLL pair<ll,ll>
#define MP make_pair
#define sqr(x) ((x)*(x))
#define lowbit(x) ((x)&(-(x)))
#define lson (ind<<1)
#define rson (ind<<1|1)
#define se second
#define fi first
#define sz(x) ((int)x.size())
#define EX0 exit(0)

typedef  long long ll;
typedef unsigned long long ull;
typedef double db;
typedef long double ld;
using namespace std;
typedef vector<ll> VLL;
typedef vector<int> VI;
const int block_size = 320;
typedef complex<ll> point;
const ll mod = 1e9 + 7;
const ll inf = 1e9 + 7;
const ll INF = 1e18 + 5;
const ld eps = 1e-9;
const db PI = atan(1) * 4;
template<typename T>
inline int sign(const T&a) {
	if (a < 0)return -1;
	if (a > 0)return 1;
	return 0;
}
string to_string(string s) {
	return '"' + s + '"';
}

string to_string(const char* s) {
	return to_string((string)s);
}

string to_string(bool b) {
	return (b ? "true" : "false");
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
	return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A>
string to_string(A v) {
	bool first = true;
	string res = "{";
	for (const auto &x : v) {
		if (!first) {
			res += ", ";
		}
		first = false;
		res += to_string(x);
	}
	res += "}";
	return res;
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
	cerr << " " << to_string(H);
	debug_out(T...);
}

#ifndef ONLINE_JUDGE
#define dbg(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define dbg(...) {}
#endif

template<typename T, typename S>inline bool upmin(T&a, const S&b) { return a > b ? a = b, 1 : 0; }
template<typename T, typename S>inline bool upmax(T&a, const S&b) { return a < b ? a = b, 1 : 0; }

template<typename T> inline void in(T &x) {
	x = 0;
	T f = 1;
	char ch = getchar();
	while (!isdigit(ch)) {
		if (ch == '-') f = -1;
		ch = getchar();
	}
	while (isdigit(ch)) {
		x = x * 10 + ch - '0';
		ch = getchar();
	}
	x *= f;
}

ll twop(int x) {
	return 1LL << x;
}

template<typename A, typename B > inline void in(A&x, B&y) {
	in(x);
	in(y);
}
template<typename A, typename B, typename C>inline void in(A&x, B&y, C&z) {
	in(x);
	in(y);
	in(z);
}
template<typename A, typename B, typename C, typename D> inline void in(A&x, B&y, C&z, D&d) {
	in(x);
	in(y);
	in(z);
	in(d);
}
ll gcd(ll a, ll b) { return b == 0 ? a : gcd(b, a%b); }

namespace SOLVE {
	

	void main() {
		int n;
		in(n);

		double inv = -1;
		vector<double> P(n + 5);
		vector<vector<double>> memo(n + 5, vector<double>(n + 5, inv));
		
		REP(i, 1, n + 1) {
			cin >> P[i]; // 1 indexed
		}
		
		// 1 coin base cases
		memo[1][0] = 1 - P[1];
		memo[1][1] = P[1];

		
		for (int i = 2; i <= n; i++) {
			// no heads base cases
			memo[i][0] = memo[i - 1][0] * (1 - P[i]);
			// all heads base cases
			memo[i][i] = memo[i - 1][i - 1] * (P[i]);
		}


		// bottom up approach!
		for (int coin = 2; coin <= n; coin++) { // 1-indexed!
			for (int heads = 1; heads < coin; heads++) {
				memo[coin][heads] = memo[coin - 1][heads] * (1 - P[coin]) + memo[coin - 1][heads - 1] * (P[coin]);
			}
		}

		double ans = 0;
		for (int i = (n / 2 + 1); i <= n; i++) {
			ans += memo[n][i];
		}
		cout << setprecision(10) << ans << endl;





	}
}


signed main() {

	int t;
	//    in(t);
	t = 1;
	while (t--) {
		SOLVE::main();

	}















	return 0;
}


Measured memory_bytes: 70912