#include <bits/stdc++.h>

#define rep(i, j, k) for (int i = (j); i <= (k); ++i)
#define per(i, j, k) for (int i = (j); i >= (k); --i)
#define SZ(v) int((v).size())
#define ALL(v) (v).begin(),(v).end()
#define fi first
#define se second
using ll = long long;
using pii = std::pair<int, int>;
using pll = std::pair<ll, ll>;

template<class T> void chkmn(T &x, T y) { if (y < x) x = y; }
template<class T> void chkmx(T &x, T y) { if (y > x) x = y; }

using namespace std;

const int maxn = 500010;

struct line {
  double k, b;
  line() { k = 0, b = -1e9; }
  line(double _k, double _b) { k = _k, b = _b; }
  double F(int x) { return x * k + b; }
} t[maxn << 2];

int n;
double a[maxn], f[maxn];

#define mid ((l + r) >> 1)

void upd(int c, int l, int r, line v) {
  if (t[c].k == 0 && t[c].b == -1e9) return t[c] = v, void();
  if (t[c].F(mid) < v.F(mid)) swap(t[c], v);
  if (v.k < t[c].k) upd(c << 1, l, mid, v);
  else upd(c << 1 | 1, mid + 1, r, v);
}

double qry(int c, int l, int r, int x) {
  if (l == r) return t[c].F(x);
  if (x <= mid) return max(qry(c << 1, l, mid, x), t[c].F(x));
  else return max(qry(c << 1 | 1, mid + 1, r, x), t[c].F(x));
}

#undef mid

int main() {
  cin.tie(nullptr) -> ios::sync_with_stdio(false);
  cin >> n;
  rep (i, 1, n) cin >> a[i];
  f[n] = n;
  upd(1, 0, n, line(1 - a[n], a[n] * f[n]));
  per (i, n - 1, 0) {
    f[i] = i;
    chkmx(f[i], qry(1, 0, n, i));
    if (i != 0) upd(1, 0, n, line(1 - a[i], a[i] * f[i]));
  }
  cout << fixed << setprecision(20) << f[0] << '\n';
}