BZOJ 4318 OSU!

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 100500
using namespace std;
int n;
double dp1[maxn],dp2[maxn],dp3[maxn],x;
int main()
{
    scanf("%d",&n);
    for (int i=1;i<=n;i++)
    {
        scanf("%lf",&x);
        dp1[i]=(dp1[i-1]+1)*x;
        dp2[i]=(dp2[i-1]+2*dp1[i-1]+1)*x;
        dp3[i]=dp3[i-1]+(3*dp2[i-1]+3*dp1[i-1]+1)*x;
    }
    printf("%.1lf\n",dp3[n]);
    return 0;
}