#include <fstream>
#include <iomanip>
using namespace std;
int n,i;
long double s,tri;
struct vct
{
    long double x,y;
}v[100005];
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");

    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>v[i].x;
        f>>v[i].y;
    }
    v[0].x=v[n].x;
    v[0].y=v[n].y;
    for(i=0;i<n;i++)
    {
        tri=(v[i].x*v[i+1].y - v[i+1].x*v[i].y)/2;
        s+=tri;
    }
    g<<fixed<<setprecision(6)<<s;


    return 0;
}