import core.bitop;
import std.algorithm;
import std.ascii;
import std.bigint;
import std.conv;
import std.functional;
import std.math;
import std.numeric;
import std.range;
import std.stdio;
import std.string;
import std.random;
import std.typecons;

alias sread = () => readln.chomp();
alias Point2 = Tuple!(long, "y", long, "x");

T lread(T = long)()
{
    return readln.chomp.to!T();
}

T[] aryread(T = long)()
{
    return readln.split.to!(T[])();
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}

void minAssign(T, U = T)(ref T dst, U src)
{
    dst = cast(T) min(dst, src);
}

void maxAssign(T, U = T)(ref T dst, U src)
{
    dst = cast(T) max(dst, src);
}

void main()
{
    long N = lread();
    auto P = aryread!double();
    auto dp = new double[][](N + 1, N + 1);

    foreach (ref row; dp)
        row[] = 0.0;

    dp[0][0] = 1.0;

    foreach (n; 0 .. N)
        foreach (x; 0 .. N)
        {
            dp[n + 1][x] += dp[n][x] * (1 - P[n]);
            dp[n + 1][x + 1] += dp[n][x] * P[n];
        }
    // foreach (row; dp)
    //     writeln(row);
    writefln("%.10f", dp[N][(N + 1) / 2 .. $].sum());
}
