/*
ID: hepic
PROG: sabotage
LANG: C++11
*/
#include <bits/stdc++.h>

#define FOR(i, a, b) for(auto i=a; i<=b; ++i)
#define REP(i, a, b) for(auto i=a; i<b; ++i)
#define FORI(i, a, b) for(auto i=a; i!=b+1-2*(a>b); i+=1-2*(a>b))
#define REPI(i, a, b) for(auto i=a-(a>b); i!=b-(a>b); i+=1-2*(a>b))
#define ALL(v) v.begin(),v.end()
#define mp(a, b) make_pair(a, b)
#define pb(a) push_back(a)
#define pf(a) push_front(a)
#define eb(a, b) emplace_back(a, b)
#define fir first
#define sec second
#define what_is(x) cout<<#x<<" is "<<x<<endl;
#define type(x) typeid(x).name()
#define ms(arr, val) memset(arr, val, sizeof(arr))
#define min3(a,b,c) min(min(a,b),c)
#define max3(a,b,c) max(max(a,b),c)
#define PI acos(-1)
#define open_read freopen("disrupt.in", "r", stdin)
#define open_write freopen("disrupt.out", "w", stdout)

using namespace std;

typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
typedef pair<double, double> PDD;
typedef pair<int, int> PII;
typedef pair<int, PII> PIPII;
typedef pair<PII, PII> PPIIPII;
typedef pair<LL, LL> PLL;
typedef pair<ULL, ULL> PUU;
typedef pair<LL, PLL> PLPLL;
typedef pair<PLL, PLL> PPLLPLL;
typedef pair<int, LL> PIL;
typedef pair<LL, int> PLI;


template<typename T, typename T1>
ostream& operator << (ostream &out, pair<T, T1> obj)
{
    out << "(" << obj.first << ", " << obj.second << ")";
    return out;
}


template<typename T, typename T1>
ostream& operator << (ostream &out, map<T, T1> cont)
{
    typename map<T, T1>::const_iterator itr = cont.begin();
    typename map<T, T1>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T>
ostream& operator << (ostream &out, set<T> cont)
{
    typename set<T>::const_iterator itr = cont.begin();
    typename set<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T>
ostream& operator << (ostream &out, multiset<T> cont)
{
    typename multiset<T>::const_iterator itr = cont.begin();
    typename multiset<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T, template<typename ELEM, typename ALLOC=allocator<ELEM>> class CONT>
ostream& operator << (ostream &out, CONT<T> cont)
{
    typename CONT<T>::const_iterator itr = cont.begin();
    typename CONT<T>::const_iterator ends = cont.end();

    for(; itr!=ends; ++itr)
        out<<*itr<<" ";
    out<<endl;

    return out;
}


template<typename T, unsigned int N, typename CTy, typename CTr>
typename enable_if<!is_same<T, char>::value, basic_ostream<CTy, CTr> &>::type
operator << (basic_ostream<CTy, CTr> &out, const T (&arr)[N])
{
     REP(i, 0, N)
        out<<arr[i]<<" ";
    out<<endl;

    return out;
}


template<typename T>
T gcd(T a, T b)
{
    T min_v = min(a, b);
    T max_v = max(a, b);

    while(min_v)
    {
        T temp = max_v % min_v;
        max_v = min_v;
        min_v = temp;
    }

    return max_v;
}


template<typename T>
T lcm(T a, T b)
{
    return (a*b) / gcd(a, b);
}


template<typename T>
T fast_exp_pow(T base, T exp, T mod)
{
    T res = 1;

    while(exp)
    {
        if(exp&1)
        {
            res *= base;
            res %= mod;
        }

        exp >>= 1;
        base *= base;
        base %= mod;
    }

    return res % mod;
}

/*#################################################################################################################
###################################################################################################################
###################################################################################################################
#################################################################################################################*/

#define SIZE 3010

int N;
double probs[SIZE], dpMax[SIZE][SIZE];


int main() {
    scanf("%d", &N);    
    
    FOR (i, 1, N) {
        scanf("%lf", probs + i);
    }
    
    dpMax[1][0] = 1.0 - probs[1];
    dpMax[1][1] = probs[1];

    FOR (i, 2, N) {
        dpMax[i][0] = dpMax[i - 1][0] * (1.0 - probs[i]);

        FOR (j, 1, i) {
            dpMax[i][j] = dpMax[i - 1][j] * (1.0 - probs[i]) + dpMax[i - 1][j - 1] * probs[i];
        }
    }
    
    double answer = 0;

    FOR (i, N / 2 + 1, N) {
        answer += dpMax[N][i];
    }

    printf("%.10f\n", answer);
    return 0;
}


Measured memory_bytes: 69120