#include <fstream>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

double arie;

double absx(double x){
  if(x < 0) {
    return -x;
  }
  return x;
}

void addTriangle(double x1, double y1, double x2, double y2) {
  arie += (x1 * y2 - x2 * y1);
}

int main() {

  int n;
  double first[5];
  cin >> n;


  double x1, x2, y1, y2;

  cin >> x1 >> y1;

  first[0] = x1;
  first[1] = y1;

  for(int i = 1; i < n; i++){
    x2 = x1;
    y2 = y1;
    cin >> x1 >> y1;
    addTriangle(x2, y2, x1, y1);
  }

  addTriangle(x1, y1, first[0], first[1]);

  double m = absx(arie) / 2;

  cout << fixed << m;

  return 0;
}