#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.y+p2.y)*(p2.x-p1.x);
        }
        p1=p2;
    }
    s+=(p1.y+po.y)*(po.x-p1.x);
    if(s<0) s=-s;
    printf("%.5lf",s/2);
}