#include<fstream>

using namespace std;

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

struct point
{
    double x,y;
};

double area = 0;
point p[100001];
int n;

int main()
{
    f>>n;
    for (int i=0;i<n;i++)
        f>>p[i].x>>p[i].y;
    p[n] = p[0];
    f.close();

    for (int i=0;i<n;i++)
        area += ( (p[i].x*p[i+1].y) - (p[i+1].x*p[i].y) );

    g<<area/2.0;
    g.close();

    return 0;
}