#include <bits/stdc++.h>
const int INF = 2147483647;
const int INF2 = 0x3f3f3f3f;
const long long INF64 = (long long)4e18;
const double INFD = 1e30;
const double EPS = 1e-9;
const double PI = std::acos(-1);
const int MOD = 1e9 + 7;
template <typename T>
inline T read() {
  T X = 0, w = 0;
  char ch = 0;
  while (!isdigit(ch)) {
    w |= ch == '-';
    ch = getchar();
  }
  while (isdigit(ch)) X = (X << 3) + (X << 1) + (ch ^ 48), ch = getchar();
  return w ? -X : X;
}
const int MAXN = 100005;
const int MAXV = 1 << 20;
int n, m, k;
int dr[4] = {0, 1, 0, -1};
int dc[4] = {-1, 0, 1, 0};
int dr2[8] = {1, 1, 1, -1, -1, -1, 0, 0};
int dc2[8] = {1, 0, -1, 1, 0, -1, 1, -1};
int CASE = 1;
char A[MAXN], B[MAXN];
double p[MAXN];
double EN[MAXN], E2[MAXN];
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%lf", &p[i]);
    EN[i] = (EN[i - 1] + 1) * p[i];
  }
  for (int i = 1; i <= n; i++) {
    E2[i] = E2[i - 1] + (2 * EN[i - 1] + 1) * p[i];
  }
  printf("%lf", E2[n]);
  return 0;
}
