#include <iostream>

using std::cout;
using std::cin;
using std::endl;

struct node
{
	int value;
	node *next;
	node* prev;
};

class List
{
	node *tail, *head;

public:
	List()
	{
		head = tail = NULL;
	}

	~List()
	{
		node *p = head->next, *q = head;
		while (p != tail)
		{
			q->next = p->next;
			p->next->prev = p->prev;
			delete p;
			p = q->next;
		}
		node *r = head;
		head->next = NULL;
		delete r;
		delete tail;
	}

	void add_tail(double data)
	{
		node *p = new node;
		p->value = data;
		if (head)
		{
			p->next = NULL;
			p->prev = tail;
			tail->next = p;
		}
		else
		{
			head = tail = p;
		}
	}

	double sum(int n)
	{
		int m = n;
		node *p = head;
		node *q = tail;
		double summa = 0.0;
		while (m >= n / 2)
		{
			summa += (p->value*q->value);
			p = p->next;
			q = q->next;
			m--;
		}
		return (summa * 2);

	}
};

int main()
{
	setlocale(LC_ALL, "Russian");
	int n = 0;
	double x;
	List list;
	while (cin >> x)
	{
		list.add_tail(x);
		n++;
	}
	if (n % 2 != 0) cout << "Error" << endl;
	else
	{
		cout << list.sum(n);
	}
	system("pause");
	return 0;
}