#include <fstream>
#include <iomanip>
#define nm 100001

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct pairs{
    double x;
    double y;
};

int n;
double rez;
pairs v[nm];

int main()
{
    int i;
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    fin.close();

    v[n+1]=v[1];
    for(i=1;i<=n;i++){
        rez+=(v[i].x*v[i+1].y -v[i+1].x*v[i].y);
    }
    fout<<fixed<<setprecision(6)<<rez/2;
    fout.close();
    return 0;
}