#include <math.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define print(n) cout << (n) << endl
#define fprint(n) cout << setprecision(16) << (n) << endl
#define ceil_div(a, b) (((a) - 1) / (b) + 1)
#define rep(i, l, n) for (int i = (l); i < (n); i++)
#define itrep(itr, st) for (auto itr = st.begin(); itr != st.end(); itr++)
#define ite(i, a) for (auto& i : a)
#define all(x) x.begin(), x.end()
#define lb(A, x) (lower_bound(all(A), x) - A.begin())
#define ub(A, x) (upper_bound(all(A), x) - A.begin())
using str = string;
using ll = long long;
using Pair = pair<int, int>;
using mint = modint1000000007;
using Mint = modint998244353;
template <class T>	using V = vector<T>;
template <class T>	using VV = V<V<T> >;
template <class T>	using PQ = priority_queue<T, V<T>, greater<T> >;
template <class T>	using PQR = priority_queue<T>;
template <class T>	using BIT = fenwick_tree<T>;
const ll INF = 4611686018427387903;
const int inf = 2147483647;
const ll mod = 1000000007;
const ll MOD = 998244353;
template <class T>  inline V<T> getList(int n) { V<T> res(n); rep(i, 0, n) { cin >> res[i]; }return res; }
template <class T>  inline VV<T> getGrid(int m, int n) { VV<T> res(m, V<T>(n)); rep(i, 0, m) { res[i] = getList<T>(n); }return res; }
template <class T> inline void prints(V<T>& vec) { if (vec.size() == 0)return; cout << vec[0];	rep(i, 1, vec.size()) { cout << ' ' << vec[i]; } cout << '\n'; }
inline V<int> dtois(string& s) { V<int> vec = {}; ite(e, s) { vec.push_back(e - '0'); } return vec; }
inline V<int> atois(string& s) { V<int> vec = {}; ite(e, s) { vec.push_back(e - 'a'); } return vec; }
inline V<int> Atois(string& s) { V<int> vec = {}; ite(e, s) { vec.push_back(e - 'A'); } return vec; }


int main(void) {
	int n;	cin >> n;
	V<int> a = getList<int>(n - 1);

	double m = 1000.0, d = 1000.0;
	ite(k, a) {
		d -= k * d / m;
	}
	printf("%.9f\n", m - d);

	return 0;
}