#include <bits/stdc++.h>   using namespace std;   const int N = 100010;   int n;   double f1[N], f2[N], f3[N], a[N], ans;   int main() {     ios::sync_with_stdio(false);     cin >> n;     for (int i = 1; i <= n; ++i) {       cin >> a[i];       f1[i] = a[i] * (f1[i - 1] + 1);       f2[i] = a[i] * (f2[i - 1] + 1 + 2 * f1[i - 1]);       f3[i] = a[i] * (f3[i - 1] + 3 * f2[i - 1] + 3 * f1[i - 1] + 1);     }     for (int i = 1; i <= n + 1; ++i) ans += (1 - a[i]) * f2[i - 1];     printf( %.10lf n , ans);     return 0;   }   