#include <bits/stdc++.h>

using namespace std;

class TestProctoring
{
public:
	double expectedTime (vector <int> p, vector <int> q)
	{
		int n = (int) (p.size ());
		int m = 1 << n;
		vector <double> w (n);
		for (int i = 0; i < n; i++)
		{
			w[i] = p[i];
			w[i] /= q[i];
		}
		double res = 0.0;
		for (int s = 1; s < m; s++)
		{
			double v = 0.0;
			double sign = -1.0;
			for (int i = 0; i < n; i++)
			{
				if (s & (1 << i))
				{
					v = v + w[i] - v * w[i];
					sign *= -1.0;
				}
			}
			res += sign * 1.0 / v;
		}
		return res;
	}
};

int main ()
{
	TestProctoring a;
	vector <int> t0 = { 2,  3,  5,  7, 11, 13, 17};
	vector <int> t1 = { 3,  5,  7, 11, 13, 17, 19};
	printf ("%.20f\n", a.expectedTime (t0, t1));
	return 0;
}