[bzoj 4318]OSU!

#include <cstdio>
using namespace std;
int main()
{
    int n;scanf("%d",&n);
    double ans,Ex,Ex2,p;
    for(int i = 1;i <= n;i++)
    {
        scanf("%lf",&p);
        ans += (3 * Ex2 + 3 * Ex + 1) * p;
        Ex2 = (Ex2 + 2 * Ex + 1) * p;
        Ex = (Ex + 1) * p;
    }
    printf("%.1lf\n",ans);
    return 0;
}