# include <fstream>
# include <iomanip>
# include <cmath>
# define DIM 100010
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double x[DIM],y[DIM],s;
int n,i;
double det(double x1,double Y1,double x2,double Y2,double x3,double Y3){
    return (x2-x1)*(Y3-Y1)-(x3-x1)*(Y2-Y1);
}
int main () {
    fin>>n;
    for(i=1;i<=n;i++){
        fin>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
        s+=det(0,0,x[i],y[i],x[i+1],y[i+1]);
    fout<<setprecision(6)<<fixed<<abs(s)/2.0<<"\n";
    return 0;
}