#include <fstream>
#include <cmath>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
struct point
{
    double x;
    double y;
} p[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);
}
int area(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;
    cin>>n;
    for(int i=1; i<=n; i++)
        cin>>p[i].x>>p[i].y;
    cout<<area(n,p);
    return 0;
}