// WA(?)

#include <iostream>
#define rep(i, l, n) for (int i = (l); i < (n); i++)
using namespace std;

int main(void) {
	int n;	cin >> n;

	float p = 1.0, m = 1000.0;
	rep(i, 0, n - 1) {
		float a;	cin >> a;
		p *= (m - a) / m;
	}
	printf("%.9f\n", m - m * p);

	return 0;
}