#include <iostream>
#include <fstream>
#include <vector>
#include <cmath>
#include <iomanip>
using namespace std;
struct Point
{
    int x,y;
};
double Area(Point hull[100001], int N)
{

  double  area=0. ;
  int i;
  for(int i=0;i<N-1;i++)
    area+=hull[i].x*hull[i+1].y-hull[i+1].x*hull[i].y;
  area+=hull[N-1].x*hull[0].y-hull[0].x*hull[N-1].y;

  return area*.5;
}
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    int n;
    f>>n;
    Point hull[100001];
    for(int i=0;i<n;i++)
        f>>hull[i].x>>hull[i].y;
    double area=Area(hull,n);
    g<<area;
    return 0;
}