#include <fstream>
#include <cmath>

using namespace std;

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

struct point
{
    int x,y;
}v[100001];
double area(point a, point b, point c)
{
    a.x -= c.x;
    b.x -= c.x;
    a.y -= c.y;
    b.y -= c.y;
    return fabs((double)(a.x * b.y - b.x * a.y) / 2);
}

double Aria(int n, point p[])
{
    double area_tot = 0;
    for (int i = 2; i < n; i++)
    {
        area_tot += area(p[1], p[i], p[i+1]);
    }
    return area_tot;
}

int main()
{
    int n;
    f >> n;
    for(int i = 1 ; i <= n ; ++i)
        f >> v[i].x >> v[i].y;
    g << Aria(n,v);
    return 0;
}