#include <iostream>
using namespace std;

int main() {
	int n;
	cin >> n;
	double sum = 0.0, temp;
	for (int i = 1; i <= n; i++) 
	{
		cin >> temp;
		sum = sum + temp * i * (n - i + 1);
	}
	printf("%.2f", sum);
	system("pause");
	return 0;
}