#include <fstream>
#include <cmath>

using namespace std;

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

struct punct
{
    double x, y;
}a[100005];

int n;

int main()
{
    f>>n;
    for(int i=1; i<=n; i++)
        f>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    long double arie=0;
    for(int i=1; i<=n; i++)
    {
        arie+=((a[i].x*a[i+1].y/2)-(a[i+1].x*a[i].y/2));
    }
    //arie/=2;
    g<<abs(arie);

    return 0;

}