#include <iostream>
#include <fstream>
#include <math.h>
#include <iomanip>
#define max 100002
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int main ()
{
    long long n,i;
    double arie=0,x[max],y[max];
    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++) arie=arie+x[i]*y[i+1]-y[i]*x[i+1];
    arie=arie/2;
    g<<fixed<<setprecision(5)<<fabs(arie);
}