#include <stdio.h>
#include <math.h>

struct point {
  int x;
  int y;
} pcoord[100000];

int main() { 
  FILE *fin = fopen( "aria.in", "r" );
  FILE *fout = fopen( "aria.out", "w" );
  int n, i;
  double x, y, xG, yG, area, a, b, c, p, x1, y1, x2, y2, x3, y3;

  fscanf( fin, "%d", &n );
  xG = yG = 0;
  for ( i = 0; i < n; ++i ) {
	fscanf( fin, "%lf%lf", &x, &y );
    pcoord[i].x = x;
	pcoord[i].y = y;
	xG += x;
	yG += y;
  }
  xG /= n;
  yG /= n;
  area = 0;
  for ( i = 0; i < n; ++i ) {
	x1 = pcoord[i].x;
    y1 = pcoord[i].y;
	x2 = pcoord[(i + 1) % n].x;
	y2 = pcoord[(i + 1) % n].y;
	x3 = xG;
	y3 = yG;
	a = sqrt( (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1) );
	b = sqrt( (x3 - x2) * (x3 - x2) + (y3 - y2) * (y3 - y2) );
	c = sqrt( (x1 - x3) * (x1 - x3) + (y1 - y3) * (y1 - y3) );
	p = (a + b + c) / 2;
	area += sqrt( p * (p - a) * (p - b) * (p - c) );
  }
  fprintf( fout, "%.5lf", area );
  fclose( fin );
  fclose( fout );
  return 0;
}