#include<stdio.h>
struct punct{double x, y;};
double rez;
punct v[3], v1;
long i, n, ac, an;

double arie(punct a, punct b)
{    return (a.x*b.y-a.y*b.x);  }

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%ld",&n);
    scanf("%lf %lf",&v[0].x,&v[0].y);
    an=0;   ac=1;
    v1.x=v[0].x;    v1.y=v[0].y;
    for (i=2;i<=n;i++)
    {
        scanf("%lf %lf",&v[ac].x,&v[ac].y);
        rez+=arie(v[an],v[ac]);
        ac=1-ac;    an=1-an;
    }
    rez+=arie(v[an],v1);
    printf("%.6lf",rez/2);
    return 0;
}