#include <iostream>
#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
 struct Point
 { double x,y; } p[100005],pct;

   double sol; int n;

 double crossprod(Point a,Point b,Point c)
 { return (double) a.x*b.y+b.x*c.y+c.x*a.y-a.y*b.x-b.y*c.x-c.y*a.x; }

int main()
{ int i;
    f>>n;
     pct.x=pct.y=1000005;

    for(i=1;i<=n;i++)
     f>>p[i].x>>p[i].y;

    n++; p[n]=p[1];

   for(i=2;i<=n;i++)
    sol+=crossprod(p[i-1],p[i],pct);

   g<<fixed<<setprecision(5)<<fabs(sol/2);
    return 0;
}