#include <bits/stdc++.h>
using namespace std;
double A[200009];
double B[200009];
int main() {
  int N, i;
  cin >> N;
  for (i = 0; i < N; i++) {
    cin >> A[i];
  }
  B[0] = A[0];
  for (i = 1; i < N; i++) {
    B[i] = (B[i - 1] + 1) * A[i];
  }
  double sum = 0;
  for (i = 0; i < N; i++) {
    sum += 2 * B[i];
    sum -= A[i];
  }
  printf("%0.15lf\n", sum);
  return 0;
}
