#include <fstream>
#include <iomanip>
using namespace std;
struct punct
{
    double x,y;
}v[100002];
int aria(punct a,punct b,punct c)
{
    return (a.x*b.y+b.x*c.y+c.x*a.y-c.x*b.y-a.x*c.y-b.x*a.y)*1.0/2;
}
int main()
{
    int n,i;
    ifstream in("aria.in");
    ofstream out("aria.out");
    in>>n;
    for(i=1;i<=n;i++)
    {
        in>>v[i].x>>v[i].y;
    }
    double solutie=0;
    for(i=3;i<=n;i++)
    {
        solutie+=aria(v[1],v[i-1],v[i]);
    }
    out << fixed << showpoint;
    out << setprecision(5);
    out << solutie << endl;
    return 0;
}