#include<cstdio>
struct punct
{
    double x,y;
};
punct p1,p2,po;
int i,n;
double s;
int main()
{
freopen("aria.in","rt",stdin);
freopen("aria.out","wt",stdout);
scanf("%ld",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%lf%lf",&p2.x,&p2.y);
        if(i==1)
        po=p2;
        else
        {
            s+=(p1.x*p2.y-p1.y*p2.x)/2;
        }
        p1=p2;
    }
    s+=(p2.x*po.y-p2.y*po.x)/2;
    printf("%.5lf",s);
}