#include <cstdio>
#include <cmath>
using namespace std;
struct POINT
{
    double x,y;
};
double aria_tr_semn(POINT P1,POINT P2,POINT P3)
{
    double arie;
    arie=0.5*((P2.x-P1.x)*(P3.y-P2.y)-(P2.y-P1.y)*(P3.x-P2.x));
    return arie;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double tempx,tempy,arie_poligon=0;
    POINT P1,P2,P3;
    scanf("%d",&n);
    scanf("%lf%lf",&tempx,&tempy);
    P1.x=tempx;
    P1.y=tempy;
    scanf("%lf%lf",&tempx,&tempy);
    P2.x=tempx;
    P2.y=tempy;
    for(i=3;i<=n;i++)
    {
        scanf("%lf%lf",&tempx,&tempy);
        P3.x=tempx;
        P3.y=tempy;
        arie_poligon=arie_poligon+aria_tr_semn(P1,P2,P3);
        P2=P3;
    }
    arie_poligon=fabs(arie_poligon);
    printf("%lf",arie_poligon);
    return 0;
}