#include <fstream>
#include <iomanip>
using namespace std;
ifstream fcin("aria.in");
ofstream fout("aria.out");
int n;
double aria;
struct pct
{
    double x,y;
};
pct v[1010001];
int main()
{
    fcin>>n;
    for(int i=1; i<=n; i++)
        fcin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1; i<=n; i++)
        aria+=(v[i].x*v[i+1].y/2.0-v[i+1].x*v[i].y/2.0);
    fout<<fixed<<setprecision(5)<<aria;
    return 0;
}