#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
 
using namespace std;
using namespace __gnu_pbds;
 
 
#define pb push_back
#define mp make_pair
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second
#define resz resize
#define ini ll_MAX
#define inl LLONG_MAX
#define MOD 1000000007
#define sz(x) ll((x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define FOR(i, a, b) for (ll i = (a); i < (b); i++)
#define F0R(i, a) for (ll i = 0; i < (a); i++)
#define FORR(i, a, b) for (ll i = a; i >= (b); i--)
#define F0RR(i, a) for (ll i = a; i >= 0; i--)
#define trav(a, x) for (auto& a : x)
#define pbds tree<ll, null_type,less<ll>, rb_tree_tag,tree_order_statistics_node_update> 
 
using ll = long long;
using vi = vector<ll>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vb = vector<bool>;
using vd = vector<double>;
using vs = vector<string>;
 
using pii = pair<ll, ll>;
using pll = pair<ll, ll>;
using pdd = pair<double, double>;
 
using vpii = vector<pii>;
using vvpii = vector<vpii>;
using vpll = vector<pll>;
using vvpll = vector<vpll>;
using vpdd = vector<pdd>;
using vvpdd = vector<vpdd>;
 


// ll Arr[105];
// ll Size[105];


// ll root (ll i)
// {
//     while(Arr[ i ] != i)
//     {
//         Arr[ i ] = Arr[ Arr[ i ] ] ; 
// i = Arr[ i ]; 
//     }
// return i;
// }


// void initialize(ll N)
// {
//     for(ll i = 0;i<N;i++)
//     {
//         Arr[ i ] = i ;
//         Size[ i ] = 1;
//     }
// }
// void weighted_union(ll A,ll B)
// {
//     ll root_A = root(A);
//     ll root_B = root(B);
//     if(Size[root_A] < Size[root_B ])
//     {
// Arr[ root_A ] = Arr[root_B];
// Size[root_B] += Size[root_A];
// }
//     else
//     {
// Arr[ root_B ] = Arr[root_A];
// Size[root_A] += Size[root_B];
// }

// }

ll mod(ll a,ll b)
{
    return (b+a%b)%b;
}
 
ll gcd(ll a,ll b)
{
    if(b==0)return a;
    else return gcd(b,a%b);
}
ll po(ll a,ll b)
{
    ll ans = 1;
    while(b--)
        ans*=a;
    return ans;
}
ll power(ll a,ll b,ll c)
{
    ll x=1,y=a;
    while(b > 0)
    {
        if(b%2 == 1)
        {
            x=(x*y)%c;
        }
        y = (y*y)%c;
        b /= 2;
    }
    return x%c;
}
 
double sqdist(double a,double b,double c,double d)
{
    return (a-c)*(a-c)+(b-d)*(b-d);
}
ll fact(ll n)
{
    if(n==0)return 1;
    ll ans = 1;
    FOR(i,1,n+1)ans*=i;
    return ans;
}


ll check(ll i)
{
    ll p = 0;
    while(i)
    {
        if(i%2)p++;
        i/=2;
    }
    // cout<<i<<' '<<p<<endl;
    return p;
}

// vector<ll> vec[100005];


void solve()
{
        
    int n;
    cin>>n;
    vector<double> vec(n+10);
    for(auto &i:vec)cin>>i;
    vector<vector<double>> h(n+10,vector<double> (n+10,0));
    auto t = h;
    h[0][1] = 1-vec[0];
    h[1][0] = vec[0];
    for(int i=2;i<=n;i++)
    {
        for(int p=0;p<=i;p++)
        {
            int q = i-p;
            // cout<<p<<' '<<q<<endl;
            h[p][q] = vec[i-1]*(p-1>=0?h[p-1][q]:0)+(1-vec[i-1])*(q-1>=0?h[p][q-1]:0);
        }
    }
    double ans = 0;
    for(int i=0;i<=n;i++)
    {
        for(int j=0;j<=n;j++)
        {
            if(i+j==n && i>=j)
                ans+=h[i][j];
        }
    }
    printf("%0.10f",ans);

    

}   
 
main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    // #ifndef ONLINE_JUDGE
    // freopen("input.txt","r",stdin);
    // freopen("output.txt","w",stdout);
    // #endif
    solve();
}

Measured memory_bytes: 145428