sub f{
	my($u,$s)=@_;
	$h{$u,$s}//=do{
		if($u==(1<<$n)-1){
			$s>0;
		}else{
			my $p=0;
			for(0..$n-1){
				if(!($u&1<<$_)){
					my $x=$a[$_];
					my $r;
					my $q=$x==1?1- f($u|1<<$_,-$s-2):($r=(1/$x)*1/(1-(1-1/$x)**2))*(1- f($u|1<<$_,-$s-$x*2)) + (1-$r)*f($u|1<<$_,$s-$x*2);
					$p=$q if $p<$q;
				}
			}
			$p;
		}
	}
}

$n=<>;
@a=glob<>;

print f(0,-1);