#include <iostream>
using namespace std;

int main() {
	double x1, y1, x2, y2, s;
	s = 0.0;
	cin >> x1, y1;
	while ( cin >> x2, y2 ) {
		s += ( x1 * y2 - x2 * y1 ) / 2;
		x1 = x2;
		y1 = y2;
	}
	if ( s < 0 ) s = - s;
	cout << s << endl;
}