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