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

int n;
double a, d = 1000;

int main()
{
	cin >> n;

	for (int i = 0; i < n - 1; i++) {
		cin >> a;
		d *= 1000 - a;
		d /= 1000;
	}

	cout << setprecision(12) << 1000 - d << endl;
}