#include <bits/stdc++.h>
#include <cstdlib>
#include <chrono>
//using namespace chrono;
#include <atcoder/all>
using namespace atcoder;


#define int long long
#define double long double
#define stoi stoll
#define endl "\n"
using std::abs;

using namespace std;


//constexpr int MOD = 1000000007;
constexpr int MOD = 998244353;
//using mint = modint1000000007;
using mint = modint998244353;

constexpr double PI = 3.14159265358979323846;
const  int INF = 1LL << 62;
const int dx[8] = { 1, 0, -1, 0 ,1,1,-1,-1};
const int dy[8] = { 0, 1, 0, -1,1,-1,1,-1 };


#define rep(i,n) for(int i=0;i<n;++i)
#define REP(i,n) for(int i=1;i<=n;i++)
#define sREP(i,n) for(int i=1;i*i<=n;++i)
#define krep(i,k,n) for(int i=(k);i<n+k;i++)
#define Krep(i,k,n) for(int i=(k);i<n;i++)
#define rrep(i,n) for(int i=n-1;i>=0;i--)
#define Rrep(i,n) for(int i=n;i>0;i--)
#define LAST(x) x[x.size()-1]
#define ALL(x) (x).begin(),(x).end()
#define MAX(x) *max_element(ALL(x))
#define MIN(x) *min_element(ALL(x)
#define RUD(a,b) (((a)+(b)-1)/(b))
#define sum1_n(n) ((n)*(n+1)/2)
#define SUM1n2(n) (n*(2*n+1)*(n+1))/6
#define SUMkn(k,n) (SUM1n(n)-SUM1n(k-1))
#define SZ(x) ((int)(x).size())
#define PB push_back
#define Fi first
#define Se second

template<class... T>
constexpr auto min(T... a) {
	return min(initializer_list<common_type_t<T...>>{a...});
}

template<class... T>
constexpr auto max(T... a) {
	return max(initializer_list<common_type_t<T...>>{a...});
}

template<class... T>
void in(T&... a) {
	(cin >> ... >> a);
}

template <class T>
using v = vector<T>;
template <class T>
using vv = vector<v<T>>;
template <class T>
using vvv = vector<vv<T>>;

using pint = pair<int, int>;
using tint = tuple<int, int, int>;

double LOG(int a, int b) {
	return log(b) / log(a);
}
double DISTANCE(int x1, int y1, int x2, int y2) {
	return sqrt(abs(x1 - x2) * abs(x1 - x2) + abs(y1 - y2) * abs(y1 - y2));
}

inline bool BETWEEN(int x, int min, int max) {
	if (min <= x && x <= max)
		return true;
	else
		return false;
}
inline bool between(int x, int min, int max) {
	if (min < x && x < max) return true;
	else return false;
}
inline bool BETWEEN2(int i, int j, int H, int W) {
	if (BETWEEN(i, 0, H - 1) && BETWEEN(j, 0, W - 1)) return true;
	else return false;
}

template<class T>
inline bool chmin(T& a, T b) {
	if (a > b) {
		a = b;
		return true;
	}
	return false;
}
template<class T>
inline bool chmax(T& a, T b) {
	if (a < b) {
		a = b;
		return true;
	}
	return false;
}

inline bool bit(int x, int i) {
	return x >> i & 1;
}

int ini() { int x; cin >> x; return x; }
string ins() { string x; cin >> x; return x; }

void  yn(bool x) {
	if (x) {
		cout << "Yes" << endl;
	}
	else {
		cout << "No" << endl;
	}
}
void  YN(bool x) {
	if (x) {
		cout << "YES" << endl;
	}
	else {
		cout << "NO" << endl;
	}
}

int ipow(int x, int n) {
	int ans = 1;
	while (n > 0) {
		if (n & 1) ans *= x;
		x *= x;
		n >>= 1;
	}
	return ans;
}

template <typename T>
vector<T> compress(vector<T>& X) {
	vector<T> vals = X;
	sort(ALL(vals));
	vals.erase(unique(ALL(vals)), vals.end());
	rep(i, SZ(X))
		X[i] = lower_bound(ALL(vals), X[i]) - vals.begin();
	return vals;
}

vector<pair<char, int>> run_length(string x) {
	vector<pair<char, int>> ans;

	char ch = x[0];
	int cou = 1;
	REP(i, SZ(x)) {
		if (x[i] == x[i - 1]) {
			cou++;
		}
		else {
			ans.push_back({ ch,cou });
			ch = x[i];
			cou = 1;
		}
	}
	return ans;
}



void solve() {
	int N = ini();
	double ans = 1000;
	rep(i, N-1) {
		int x = ini();
		ans *=  (1000 - x)/1000.0;
	}
	cout << 1000.0-ans << endl;
}

signed main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cout << fixed << setprecision(15);
	solve();
}