#include <fstream>
#include <assert.h>
#include <iomanip>

int main() {
	std::ifstream cin("aria.in");
	std::ofstream cout("aria.out");
	std::ios::sync_with_stdio(false);

	int n, x, y, x_1, y_1, sum = 0;

	cin >> n;
	assert(1 <= n && n <= 100000);
	
	for (; n ; --n) {
		cin >> x >> y;
		cin >> x_1 >> y_1;
		sum += (x * y_1 - x_1 * y);
	}

	cout << std::fixed << std::setprecision(5) << sum / 2 << '\n';

	return 0;
}