#include <fstream>
#include <vector>
#include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
struct point
{
    long double x,y;
};
inline long double crprod(point ori,point m,point n)
{
    return (m.x-ori.x)*(n.y-ori.y)-(m.y-ori.y)*(n.x-ori.x);
}
vector<point> v;
int n;
long double sol;
int main()
{
    fin>>n;
    v.resize(n);
    for(int i=0;i<n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    v.emplace_back(v[0]);
    point o;
    o.x=0;
    o.y=0;
    for(int i=0;i<=n;i++)
    {
        sol+=crprod(o,v[i],v[i+1])/2;
    }
    fout<<setprecision(6)<<fixed<<sol;
    fin.close();
    fout.close();
    return 0;
}