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

int main()
{
	int N;
	cin >> N;
	vector<int> A( N - 1 );
	for( int i = 0; i < N - 1; i++ ) cin >> A[i];

	double ans = 1000;
	for( int i = 0; i < N - 1; i++ ) {
		ans *= (1000.0 - A[i]) / 1000;
	}
	ans = 1000 - ans;

	cout << fixed << setprecision(12) << ans << endl;
}