#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 5;
int n;
double p[maxn], dp[maxn];
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%lf", &p[i]);
  double sol = 0.;
  dp[0] = 0;
  for (int i = 1; i < n; i++) {
    dp[i] = (dp[i - 1] + p[i - 1]) * p[i];
    sol += 2. * dp[i];
  }
  for (int i = 0; i < n; i++) sol += p[i];
  printf("%.9lf\n", sol);
  return 0;
}
