#include <bits/stdc++.h>
using namespace std;
double dp[100002];
int main() {
  double res = 0, prob = 0;
  int n;
  cin >> n;
  dp[0] = 0;
  for (int i = 0; i < n; i++) {
    double p;
    cin >> p;
    dp[i] = dp[i - 1] * p;
    res = res + p + dp[i] * 2.0;
    dp[i] += p;
  }
  printf("%.7lf\n", res);
  return 0;
}
