#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
const int NMAX = 1e5;

ifstream fin( "aria.in" );
ofstream fout( "aria.out" );

double x[NMAX+1];
double y[NMAX+1];
int main() {
  int n, i;
  double s = 0;
  fin >> n;
  for( i = 0; i < n; i++ )
    fin >> x[i] >> y[i];
  x[n] = x[0];
  y[n] = y[0];
  for( i = 0; i < n; i++ )
    s += x[i] * y[i+1] - x[i+1] * y[i];
  fout << setprecision(5) << s / 2;
  return 0;
}