#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, j ;
  long double s ;
  fscanf (fin, "%d", &n ) ;
  for (i = 0 ; i < n ; i++)
    fscanf (fin, "%lf %lf", &v[i].x, &v[i].y ) ;
  s = 0 ;
  j = 1 ;
  for (i = 0 ; i < n ; i++ ) {
    if (j >= n )
      j-= n ;
    s += ((long double)((long double) v[i].x * (long double)v[j].y ) - (long double) ( (long double)v[j].x * (long double)v[i].y ) ) ;
    j++;
  }
  fprintf (fout, "%.8Lf", s/2 ) ;
  return 0;
}