#include <fstream>
#include <iomanip>
using namespace std;

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

int n, i;
double A;
struct punct{

    double x;
    double y;

}a[100010];
double arie( punct O, punct a, punct b)
{
    double A = (a.x - O.x) * (b.y - a.y) - (a.y - O.y) * (b.x - a.x);
    return A / 2;
}
int main()
{
    fin >> n;
    for(i = 1; i <= n;i ++)
        fin >> a[i].x >> a[i].y;
    for(i = 1; i < n; i ++)
        A += arie(a[1],a[i],a[i + 1]);
    fout << fixed << setprecision(6) << A;
    return 0;
}