#include <iostream>
#include <complex>
#include <cmath>
using namespace std;

int main() {
	typedef complex<double> xy_t;
	double x1, y1, x2, y2, s;
	xy_t z1, z2;
	s = 0.0;
	cin >> x1, y1;
	while ( cin >> x2, y2 ) {
		z1 = xy_t( x1, y1 );
		z2 = xy_t( x2, y2 );
		s += ( conj( z1 ) * z2 ).real();
		x1 = x2;
		y1 = y2;
	}
	if ( s < 0 ) s = - s;
	cout << s << endl;
}