#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i, cnt;
  double ans, a, dp[100005] = {0}, pr[100005] = {0};
  cin >> n;
  for (i = 1; i <= n; i++) {
    cin >> a;
    pr[i] = pr[i - 1] * a + a;
    dp[i] = dp[i - 1] + a * (2 * pr[i - 1] + 1);
  }
  printf("%.15lf", dp[n]);
  return 0;
}
