#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <cstring>
#include <cassert>
#include <list>
#include <map>
#include <unordered_map>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <utility>
#include <bitset>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <limits>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<long long> vll;
#define left(x) (x << 1)
#define right(x) (x << 1) + 1
#define mid(l, r) ((l + r) >> 1)
#define mp make_pair
#define pb push_back
#define all(a) a.begin(), a.end()
#define debug(x)                          \
    {                                     \
        cerr << #x << " = " << x << "\n"; \
    }
#define debug2(x, y)                                                  \
    {                                                                 \
        cerr << #x << " = " << x << ", " << #y << " = " << y << "\n"; \
    }
#define debug3(x, y, z)                                                                           \
    {                                                                                             \
        cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << "\n"; \
    }
#define debug4(x, y, z, w)                                                                                                    \
    {                                                                                                                         \
        cerr << #x << " = " << x << ", " << #y << " = " << y << ", " << #z << " = " << z << ", " << #w << " = " << w << "\n"; \
    }
#define ss second
#define ff first
#define m0(x) memset(x, 0, sizeof(x))

template <typename T>
T min_(T a, T b)
{
    return a > b ? b : a;
}
template <typename T, typename... Ts>
T min_(T first, Ts... last) { return min_(first, min_(last...)); }
template <typename T>
T max_(T a, T b) { return a > b ? a : b; }
template <typename T, typename... Ts>
T max_(T first, Ts... last) { return max_(first, max_(last...)); }

template <class T, class S>
std::ostream &operator<<(std::ostream &os, const std::pair<T, S> &t)
{
    os << "(" << t.first << ", " << t.second << ")";
    return os;
}
template <typename T>
ostream &operator<<(ostream &out, const vector<T> &v)
{
    out << "[";
    size_t last = v.size() - 1;
    for (size_t i = 0; i < v.size(); ++i)
    {
        out << v[i];
        if (i != last)
            out << ", ";
    }
    out << "]";
    return out;
}

ll pwr(ll base, ll p, ll mod)
{
    ll ans = 1;
    while (p)
    {
        if (p & 1)
            ans = (ans * base) % mod;
        base = (base * base) % mod;
        p /= 2;
    }
    return ans;
}
ll gcd(ll a, ll b) { return b == 0 ? a : gcd(b, a % b); }
ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

const int day[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
const int digarr[10] = {6, 2, 5, 5, 4, 5, 6, 3, 7, 6};
const int dx[4] = {0, 1, 0, -1};
const int dy[4] = {1, 0, -1, 0};
const int dxo[8] = {-1, -1, -1, 0, 1, 1, 1, 0};
const int dyo[8] = {-1, 0, 1, 1, 1, 0, -1, -1};
const long double PI = (long double)(3.1415926535897932384626433832795);
const ll mx_ll = numeric_limits<ll>::max();
const int mx_int = numeric_limits<int>::max();
const int mod = 1e9 + 7;
const int maxn = 2e5 + 7;
void cppio()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
}

double dp[5001][5001];
void solve()
{
    int n; cin >> n;
    vector<double> p(n+1);
    for(int i = 0 ; i < n; i++)
    {
        cin >> p[i+1];
    }
    dp[0][0] = 1;
    for(int i = 1; i <= n; i++)
    {
        for(int h = 0; h <= i; h++)
        {
            dp[i][h] = ((h == 0) ? 0 : dp[i-1][h-1] * p[i]) + dp[i-1][h] * (1 -p[i]);
        }
    }
    double ans = 0;
    for(int h = 0; h <= n; h++)
    {
        if(h > (n-h))
            ans += dp[n][h];
    }
    cout << setprecision(10)  << fixed << ans << "\n";
}

int main()
{
    cppio();
#ifdef __APPLE__
    //freopen("test.in", "rb", stdin);
    //freopen("  .out", "wb", stdout);
#endif

    int num_test_cases = 1;
    // cin >> num_test_cases;
    for (int t = 1; t <= num_test_cases; ++t)
    {
        solve();
    }
    return 0;
}


Measured memory_bytes: 117248