#include <bits/stdc++.h>
using namespace std;
int main() {
  double ans, p, q, p1, p2, p3;
  int n, i;
  scanf("%d", &n);
  ans = 0.0;
  q = p2 = p3 = 1;
  p1 = 0;
  for (i = 1; i <= n + 1; i++) {
    if (i <= n)
      scanf("%lf", &p);
    else
      p = 0.0;
    ans += (1 - p) * p1;
    p1 = (p1 + p2) * p;
    q = 1.0 - p;
    p2 = (p2 + 2.0 * p3) * p + q;
    p3 = p3 * p + q;
  }
  printf("%.15f\n", ans);
  return 0;
}
