#include <iostream>
#include <fstream>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct punct{

            long double x,y;

            };
punct p[100010];
int n,i;
long double s;
long double arie(punct a, punct b, punct c)
{
    return (a.x*b.y+b.x*c.y+a.y*c.x-b.y*c.x-a.y*b.x-a.x*c.y)/2;
}

int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>p[i].x>>p[i].y;
    for(i=2;i<n;i++)
    {
        s=s+arie(p[1],p[i],p[i+1]);
    }
    g<<fixed<<setprecision(5)<<s;
}