#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
long double x[100011],y[100011],sol;
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];
    for(i=1;i<=n;i++)
        sol=sol+(x[i]*y[i+1]-x[i+1]*y[i]);
    g<<setprecision(6)<<fabs(sol/2.0);
    return 0;
}