#include <bits/stdc++.h>   using namespace std;   const double eps = 1e-8;   double e[100010], r[100010], p[100010];   int main() {     int n;     {       scanf( %d , &n);       for (int i = 0; i < n; i++) {         scanf( %lf , &p[i]);       }       e[0] = p[0];       r[0] = p[0];       for (int i = 1; i < n; i++) {         e[i] = e[i - 1] + p[i] + 2.0 * p[i] * r[i - 1];         r[i] = (r[i - 1] + 1.0) * p[i];       }       printf( %.8lf n , e[n - 1]);     }     return 0;   }   