#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;
	long double sum = 0;

	cin >> n;
	assert(1 <= n && n <= 100000);
	
	long double x[n], y[n];

	for (int i = 0 ; i < n ; ++i) {
		cin >> x[i] >> y[i];
	}
	x[n] = x[0];
	y[n] = y[0];
	for (int i = 0 ; i < n ; ++i) {
		sum += x[i] * y[i + 1] - x[i + 1] * y[i];
	}

	cout << std::fixed << std::setprecision(7) << sum / 2.0 << '\n';

	return 0;
}