#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

int n;
 double arie;

struct punct{double x;double y;}v[100002];

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);


scanf("%d",&n);
for(int i=1; i<=n; i++)
scanf("%lf%lf",&v[i].x,&v[i].y);
v[n+1].x=v[1].x;
v[n+1].y=v[1].y;

//p=a;
for(int i=1; i<=n; i++)
{
    //scanf("%lf%lf",&b.x,&b.y);

    //arie+=f(a,b);
//arie+=a.x*b.y-b.x*a.y;

  arie=arie+v[i].x *v[i+1].y - v[i+1].x*v[i].y;
  //  a=b;

}

    //arie+=f(a,p);

   printf("%.6lf",arie/2.0);
    return 0;
}