#include <bits/stdc++.h>
double A[200000], B[200000], C[200000], p[200000], F[200000];
int i, j, n, m;
int main() {
  scanf("%d", &n);
  double now, last = 0;
  C[0] = 1;
  for (i = 1; i <= n; i++) {
    if (i <= n) scanf("%lf", &now);
    C[i] = now * (C[i - 1]) + 1 - now;
    B[i] = now * (B[i - 1] + C[i - 1]);
    A[i] = (A[i - 1] + 2 * B[i - 1] + C[i - 1]) * now + (1 - now) * A[i - 1];
    last = now;
  }
  printf("%.9lf\n", A[n]);
  return 0;
}
