#include <fstream>
#include <iomanip>

struct point
{
	int x;
	int y;
};

point a[1000001];

int main( int argc, char* argv[] )
{
	std::ifstream input("aria.in");
	std::ofstream output("aria.out");

	int N;
	input >> N;
	for ( int i = 0; i < N; ++i )
	{
		int first, second;
		input >> first >> second;
		a[i].x = first; a[i].y = second;
	}
	float sum = 0.0f;
	for ( int i = 0; i < N-1; ++i )
	{
		sum += ( a[i].x * a[i+1].y - a[i+1].x * a[i].y );
	}
	output << std::setprecision(5) << sum / 2 << "\n";

	input.close();
	output.close();
	return 0;
}