// This program was written by Mircea Rebengiuc
// on 03.08.2022
// for problem aria

#include <stdio.h>
#include <ctype.h>

FILE *fin, *fout;

#define MAXN 100000

double x[MAXN], y[MAXN];

int main(){
  fin  = fopen( "aria.in",  "r" );
  fout = fopen( "aria.out", "w" );

  int n, i, j;
  double aria = 0;
  
  fscanf( fin, "%d", &n );
  for( i = 0 ; i < n ; i++ )
    fscanf( fin, "%lf%lf", x + i, y + i );

  for( i = 0 ; i < n ; i++ ){
    j = (i + 1) % n;

    aria += (x[j] - x[i]) * (y[i] + y[j]);
  }

  fprintf( fout, "%.6lf\n", (aria < 0 ? -aria : aria) / 2 );

  fclose( fin );
  fclose( fout );
  return 0;
}