#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
struct point
{
    double x,y;
}p[100001];
double arie(point a, point b, point c) {
a.x -= c.x; b.x -= c.x;
a.y -= c.y; b.y -= c.y;
return (a.x * b.y - b.x * a.y) / 2;
}
int aria(int n, point p[])
{
    double ar=0;
    for(i=2;i<n;i++)
        ar+=arie(p[1],p[i],p[i+1]);
    return ar;
}
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>p[i].x>>p[i].y;
    }
    g<<aria(n,p);
    return 0;
}