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

                            #define FOR(i,a,b) for(int i = (a); i < (b); i++)
                            #define rep(i,n) FOR(i,0,n)
                            #define RFOR(i,b,a) for(int i = (b) - 1; i>= (a); i--)
                            #define ITER(it, a) for(typeof(a.begin()) it = a.begin(); it != a.end(); it++)
                            #define FILL(a, value) memset(a, value, sizeof(a)

                            #define SZ(a) (int)a.size()
                            #define ALL(a) a.begin(), a.end()
                            #define PB push_back
                            #define MP make_pair
                            #define er erase
                            #define in insert
                            #define f first
//                            #define s second



                            typedef  long long LL;
                            typedef vector<int> VI;
                            typedef vector<LL > VL;
                            typedef vector<vector<int > > VVI;
                            typedef vector<vector<LL>> VVL;
                            typedef vector <vector<pair<int, int >> >VVPI;
                            typedef vector<vector<pair<LL, LL >> >VVPL;
                            typedef vector<pair<LL, LL >> PL;
                            typedef vector<PL> VPL;
                            typedef pair<int, int> PII;
                            typedef vector<PII> VPI;

                            const double PI = acos(-1.0);
                            const int INF = 1e9;
                            const LL LINF = (LL)1e18;

                            const double EPS = 1e-7;
                            const int MAS =  3 *1e5 + 10;
                            const int mod = 998244353;
                            const int MOD =1e9+7;
                            const int LOG= 19;
                            const int ar = 1e5 +100;
                            const int MAX = 1e6 + 1e2;
                            double dp[3010][3010];
                        int main() {
                            ios_base::sync_with_stdio(false);
                            cin.tie(NULL);
                            double ans =0.0;
                            int n;
                            cin >> n;
                            vector<double> a(n + 1);
                            rep(i,n)cin >> a[i + 1];
                            dp[1][0] = 1.0 - a[1];
                            dp[1][1] = a[1];
                            FOR(i,2,n + 1)
                            {
                                FOR(cnt,0,i + 1)
                                {
                                    if(cnt == 0)
                                    {
                                        dp[i][cnt] = dp[i-1][cnt] * (1 - a[i]);
                                        continue;
                                    }
                                    //if(cnt == 1 && i == 2)
                                  //  {
                                     //   cout << dp[i-1][cnt] <<' '<<dp[i-1][cnt-1] <<' ' << a[i] << endl;
                                   // }
                                    dp[i][cnt]+=dp[i-1][cnt] * ( 1.0 - a[i]);
                                    dp[i][cnt]+=dp[i-1][cnt - 1] * (a[i]);
                                }
                            }
                          //  cout << dp[2][1] <<' ';

                            //rep(i,cnt)
                            int nd = n/2  + 1;
                            FOR(cnt,nd,n + 1)
                            {
                                ans+=dp[n][cnt];
                            }
                            cout << fixed << setprecision(9) << ans;

                            


                        }

















































Measured memory_bytes: 69120