#include <cstdio>
#include <cmath>
#define eps 1.e-6
#define INF 1.e9

using namespace std;

class POINT
{
    private:
        double x,y;
    public:
        void set(double a,double b)
        {
            x=a;y=b;
        }
        double distanta(const POINT & other)
        {
            return sqrt((x-other.x)*(x-other.x)+(y-other.y)*(y-other.y));
        }
        friend double aria(POINT & a,POINT & b,POINT & c)
        {
            double l1,l2,l3,p;
            l1=a.distanta(b);l2=b.distanta(c);l3=c.distanta(a);
            p=(l1+l2+l3)/2;
            return sqrt(p*(p-l1)*(p-l2)*(p-l3));
        }
};
POINT v[100005];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double a,b,s=0.0;
    scanf("%d",&n);
    for(i=1;i<=n;++i)
    {
        scanf("%lf%lf",&a,&b);
        v[i].set(a,b);
    }
    for(i=3;i<=n;++i)
        s+=aria(v[1],v[i-1],v[i]);
    printf("%.5lf",s);
    return 0;
}