#include <fstream>
#include <iomanip>
using namespace std;
struct punct
{
    double x,y;
}v[100002];
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;
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
    {
        solutie=solutie+v[i].x*v[i+1].y-v[i].y-v[i+1].x;
    }
    out << fixed << showpoint;
    out << setprecision(5);
    out << solutie << endl;
    return 0;
}