#include <fstream>
#include <vector>
#include <iomanip>
#define x first
#define y second
#define pp pair<double,double>
using namespace std;
vector <pp> v;
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    int n,i;double x,y,A=0.0;
    f>>n;
    for (i=1;i<=n;i++)
    {
        f>>x>>y;
        v.push_back(make_pair(x,y));
    }
    for (i=1;i<n-1;i++)
    {
        A+=(v[i-1].x*v[i].y-v[i-1].y*v[i].x);
    }
    g<<setprecision(20)<<A<<'\n';
    f.close();
    g.close();
    return 0;
}