#include <iostream>
using namespace std;

int main() {
	double sum = 0 , a_first, a_prev, a_cur;
	cin >> a_first;
    a_cur = a_first;
    a_prev = a_first;
    
	while (a_cur >= 0) {
		cin >> a_cur;
		if (a_cur < 0) {
			sum += a_prev*a_first;
			break;
		}
		sum += a_prev*a_cur;
		a_prev = a_cur;
	}
	
	cout << "Sum = " << sum << endl;
	
	return 0;
}