#include <bits/stdc++.h>

using namespace std;
using ll = long long;

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

const int DIM = 1e5;

struct Point {
  long double x, y;
} p[DIM];

int main() {
  ios_base::sync_with_stdio(0);
  fin.tie(0);
  int n;

  fin >> n;
  for ( int i = 0; i < n; ++i ) {
	fin >> p[i].x >> p[i].y;
  }
  long double sum = 0.0;
  for ( int i = 0; i < n; ++i ) {
	sum += p[i].x * p[(i + 1) % n].y - p[(i + 1) % n].x * p[i].y;
  }
  sum = abs(sum) / 2.0;
  fout << setprecision(6) << fixed << sum;
  fin.close();
  fout.close();
  return 0;
}