#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  double x = 0, ans = 0, p[100005] = {0};
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> p[i];
  for (int i = 1; i <= n; i++) {
    x = x * p[i] + p[i];
    ans += 2 * x;
    ans -= p[i];
  }
  cout << fixed << setprecision(10);
  cout << ans;
  return 0;
}
