#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,i;
double s,x[100005],y[100005];
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++)
      s+=x[i+1]*y[i]-x[i]*y[i+1];
    if(s<0)
      s*=-1;
    g<<fixed<<setprecision(5)<<s/2;
    return 0;
}