#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:100000000,100000000")
using namespace std;
const long long inf = 1e9 + 7;
const long long mod = 1e9 + 7;
const double eps = 1e-9;
const double PI = 2 * acos(0.0);
const double E = 2.71828;
int main(void) {
  long long n;
  cin >> n;
  double ans = 0, e = 0;
  for (int i = 0; i < n; i++) {
    double temp;
    cin >> temp;
    ans += temp * (2.0 * e + 1);
    e = temp * (e + 1);
  }
  cout << setprecision(8) << fixed << ans << endl;
  return 0;
}
