#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define ll long long
#define rep(i,n) for (int i = 0; i < (n); i++)
#define coutf(f) cout << fixed << setprecision(f)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()

int main() {
	int n;
	cin >> n;
	vector<double> a(n - 1);
	rep(i, n - 1) cin >> a[i];
	double cnt = 1;
	rep(i, n - 1) cnt *= (1000 - a[i]) / 1000;
	coutf(10) << 1000 - 1000 * cnt << endl;
	return 0;
}