#include <cstdio>
double arie(double x1,double y1,double x2,double y2,double x,double y)
{
    double res=(double)(x2-x1)*(y-y1)-(double)(x-x1)*(y2-y1);
    res/=2;
    return res;
}
int main()
{
    freopen ("aria.in","r",stdin);
    freopen ("aria.out","w",stdout);
    int n;
    double a=0;
    scanf("%d",&n);
    double x1,x2,y1,y2,xx,yy;
    scanf("%lf%lf",&x1,&y1);
    xx=x1;
    yy=y1;
    for(int i=1;i<n;i++)
    {
        scanf("%lf%lf",&x2,&y2);
        double x=(arie(0,0,x1,y1,x2,y2));
        a+=x;
        x1=x2;
        y1=y2;
    }
    double x=arie(0,0,x1,y1,xx,yy);
    a+=x;
    printf("%.5lf",a);
}