#include <iostream>
using namespace std;

int main() {
	int n;
	double sum = 0;
	cin >> n;
	double x1, x2, x3;
	cin >> x2 >> x3;
	
	for (int i = 0; i < n-2; i++ ){
		x1 = x2;
		x2 = x3;
		cin >> x3;
		sum += (x1 + x2 + x3) * x2;
	}
	
	cout << "sum" << " = " << sum;
	
	return 0;
}