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

  double  area=0. ;
  int     i, j=N-1  ;

  for (i=0; i<N; i++) {
    area+=(hull[j].x+hull[i].x)*(hull[j].y-hull[i].y); j=i; }

  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;
    g<<abs(Area(hull,n));
    return 0;
}