PAT 1104

#include<cstdio>
#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<queue>
#include<map>
#include<set>
using namespace std;



int main()
{

	//freopen("input.txt","r",stdin);
	int n;
	cin>>n;

	double a[100100];
	for(int i=0;i<n;i++)
	{
		cin>>a[i];
	}
	double sum=0;
	for(int i=0;i<n;i++)
	{
		sum+=a[i]*(n-i)*(i+1);
	}
	printf("%.2lf",sum);
	//freopen("con","r",stdin);

	system("pause");
	return 0;
}