// G. Hagopian--Kattis QALY

#include <iostream>
using namespace std;

double product(double q, double y) {
	return q * y;
}

int main() {
	int N{};
	double q{}, y{}, sumProduct{};
	cin >> N;
	while (N--) {
		cin >> q >> y;
		sumProduct += product(q, y);
	}
	cout << sumProduct;
}