#include <fstream>
std::ifstream f ("aria.txt");
std::ofstream g ("aria.out");
std::pair<int, int> V[100005];
int n;
double A;
int main()
{
	f >> n;
	for(int i = 0 ; i < n; i++)
		f >> V[i].first >> V[i].second;
	V[n] = V[0];
	for(int i = 0; i < n; i++)
		A = A + (V[i].first * V[i+1].second - V[i+1].first * V[i].second);
	g << A / 2.00;
	return 0;
}