#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double x[100002],y[100002];
int main()
{   f>>n;
    for(i=1;i<=n;i++)
        f>>x[i]>>y[i];
    x[n+1]=x[1];y[n+1]=y[1];
    double S=0;
    for(i=1;i<=n;i++) S+=x[i]*y[i+1]-x[i+1]*y[i];
    if(S<0) S=-S;
    g<<fixed<<setprecision(5)<<S/2;
    g.close(); return 0;
}