#include <fstream>
#include <iomanip>
#define DIM 100002
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct punct
{
    double x,y;
}v[DIM];

int n;
double arie;
int main()
{
    f>>n;
    for(int i=1;i<=n;++i){
        f>>v[i].x>>v[i].y;
    }
    v[n+1]=v[1];
    for(int i=1;i<=n;++i){
        arie+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    }
    g<<setprecision(5)<<arie/2;
    f.close();
    g.close();
    return 0;
}