#include <cstdio>

using namespace std;
double a,b,c,d,x,y,n,s,i;
double arie(double x,double y, double a,double b,double c,double d)
{
   return (x*b+y*c+a*d-b*c-a*y-d*x)/2;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%lf",&n);
    scanf("%lf %lf %lf %lf",&x,&y,&a,&b);
    for(i=1;i<=n-2;++i)
    {
    scanf("%lf %lf",&c,&d);
    s+=arie(x,y,a,b,c,d);
    a=c;
    b=d;
    }
    printf("%lf\n",s);
    return 0;
}