#include <fstream>
#include <cmath>

using namespace std;

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

struct coord { int x,y; };
int n;
coord a[100001];

    int determinant (int x1, int y1,int x2,int y2)
      {
          int aux;
           aux = abs(((x1*y2)-(x2*y1))*0.5);
           return aux;
      }


int main()
{
    f >> n;
    for (int i=0;i<n;i++)
        f >> a[i].x >> a[i].y;
    a[n] = a[0];

    long aria = 0;
    for (int i=0;i<n;i++)
    {
        aria += determinant(a[i].x,a[i].y,a[i+1].x,a[i+1].y);
    }
    g << aria;

    return 0;
}