#define cross(a1, b1, a2, b2) ((a1)*(b2)-(a2)*(b1))
int main(){
    double a[100],b[100],s,t,x;
    int n,c=1,i;
    for(;~scanf("%d",&n);printf("%.1lf\n",fabs(s-t)),c++){
        for(i=0;i<n;i++)scanf("%lf%lf",a+i,b+i);
        for(i=s=t=0;i<n-1;i++){
            if((x=cross(a[i],b[i],a[i+1],b[i+1]))>0)s+=fabs(x)/2.0;
            else t+=fabs(x)/2;
        }
        if((x=cross(a[i],b[i],a[0],b[0]))>0)s+=fabs(x)/2;
        else t+=fabs(x)/2;
    }
exit(0);}