Code:
#include
#include
using namespace std;
const int maxn = 1000000 + 4;
double f[maxn], g[maxn], h[maxn];
int main()
{
int n;
scanf("%d",&n);
for(int i = 1;i <= n; ++i)
{
double perc;
scanf("%lf",&perc);
h[i] = (h[i - 1] + 1) * perc;
g[i] = (g[i - 1] + 2 * h[i - 1] + 1) * perc;
f[i] = f[i - 1] + (3 * (h[i - 1] + g[i - 1]) + 1) * perc;
}
printf("%.1lf",f[n]);
return 0;
}