#include<bits/stdc++.h>

using namespace std;   

#define forn(i, n) for(int i = 0; i < int(n); i++)
#define fore(i, l, r) for(int i = int(l); i < int(r); i++)
#define sz(a) int((a).size())

typedef long double ld;

int n;
vector<ld> p;


bool read()
{
	if (!(cin >> n))
		return false;
	p.resize(n + 1, .1);
	fore (i, 0, n)
		cin >> p[i + 1];
	return true;
}

const ld INF = 1e30;
const ld EPS = 1e-12;

struct line {
	ld k, b, x;
	line(ld k = .0, ld b = .0, ld x = -INF) : k(k), b(b), x(x) {}
};
ld getX(const line &a, const line &b) {
	return ld(b.b - a.b) / (a.k - b.k);
}
struct hull {
	vector<line> h;
	int u;
	hull() : h(), u(0) {}
	void insert(line l) {
		while (sz(h) > 1 && h.back().x >= getX(h.back(), l))
			h.pop_back();
		l.x = sz(h) > 0 ? getX(h.back(), l) : -INF;
		h.push_back(l);
		
//		cerr << "h = " << endl;
//		fore (i, 0, sz(h))
//			cerr << h[i].k << " " << h[i].b << " " << h[i].x << endl;
	}
	ld getVal(int x) {
		int l = -1, r = sz(h);
		while (r - l > 1) {
			int mid = (l + r) >> 1;
			if (h[mid].x > x)
				r = mid;
			else
				l = mid;
		}
		return h[l].k * x + h[l].b;
	}
};

void solve()
{
	vector<ld> d(n + 1);
	hull h;
	h.insert(line(-1, 0));
	for (int i = n; i >= 0; i--) {
		d[i] = h.getVal(-i);
//		cerr << "d[i]  = " << d[i] << endl;
		h.insert(line(-(1 - p[i]), p[i] * d[i]));
	}
	cout << d[0] << endl;
}

int main()
{
#ifdef _DEBUG
	freopen("input.txt", "r", stdin);
#endif
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout << fixed << setprecision(12);
	cerr << fixed << setprecision(12);
	int t = 1;
	//cin >> t;
	forn(i, t)
	{
	 	read();
		solve();
	}
}