#include <bits/stdc++.h>
using namespace std;
double p[100010];
double E[100010];
int main() {
  int n;
  cin >> n;
  p[0] = 0;
  E[0] = 0;
  double ans = 0;
  for (int i = 1; i <= n; ++i) {
    cin >> p[i];
    E[i] = (1.0 - p[i - 1]) * p[i] + 2 * p[i] * p[i - 1] + p[i] * E[i - 1];
    ans += E[i];
  }
  cout << setprecision(10) << ans << endl;
}
