#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
struct pct
{
   long double x,y;

}vect[100005];
long double determinant(pct a,pct b)
{
    long double val=(a.x*b.y)-(b.x*a.y);
    return val;

}
void calculezarie()
{
    long double arie=0;
    for(int i=0;i<n-1;i++)
    {
        arie+=determinant(vect[i], vect[i+1]) / 2;

    }
    arie=arie+determinant(vect[n-1], vect[0]) / 2;
   g<<abs(arie);
}
int main()
{
   f>>n;
    for(int i=0;i<n;i++)
    {
        f>>vect[i].x>>vect[i].y;
    }
    calculezarie();

    return 0;
}