#include <iostream>
#include <fstream>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct punct{
    int x;
    int y;
}*a;

int n;

long long cross(int i)
{
    punct ab,ac;

    ab.x = a[i].x-a[1].x;
    ab.y = a[i].y-a[1].y;

    ac.x = a[i+1].x-a[1].x;
    ac.y = a[i+1].y-a[1].y;

    return ab.x*ac.y-ab.y*ac.x;

}

int main()
{
    fin>>n;
    a= new punct[n+1];

    for(int i=1;i<=n;i++)
        fin>>a[i].x>>a[i].y;

   //starting calculating areas of triangles that have one point from a[0];

    double area=0;

    for(int i=2;i<n;i++)
        area+=cross(i);

    fout<<area/2;

    return 0;
}