#include <iostream>
#include <fstream>
#include <cmath>

using namespace std;

int main(int argc, char *argv[])
{

  ifstream f("aria.in");
  ofstream g("aria.out");
  int n;

  f >> n;

  double area = 0.0;
  double x, y, prev_x, prev_y, first_x, first_y;
  f >> prev_x >> prev_y;
  first_x = prev_x;
  first_y = prev_y;
  for(int i = 0; i < n - 1; i++) {
    f >> x >> y;
    area += 0.5 * (x * prev_y - prev_x * y);
    prev_x = x;
    prev_y = y;
  }

  area += 0.5 * (first_x * prev_y - first_y * prev_x);

  g << abs(area);
  return 0;
}