#include <iostream>
#include <stdio.h>
#include <stdlib.h>

#define NMAX 100000

using namespace std;

struct coord {
  double x ;
  double y ;
};

coord v [ NMAX + 1 ] ;

int main() {

  FILE *fin, *fout ;
  fin = fopen ("aria.in", "r" ) ;
  fout = fopen ("aria.out", "w" ) ;
  int n, i ;
  double s ;
  fscanf (fin, "%d", &n ) ;
  for (i = 0 ; i < n ; i++)
    fscanf (fin, "%lf %lf", &v[i].x, &v[i].y ) ;
  s = 0 ;
  for (i = 0 ; i < n ; i++ )
    s += (v[i].x * v[(i+1)%n].y ) - ( v[(i+1)%n].x * v[i].y ) ;
  fprintf (fout, "%.6f", s/2 ) ;
  return 0;
}