#include <iostream>
#include <fstream>
#include <math.h>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int main ()
{
    long long n,i;
    double arie=0,x,y,x2,y2,a,b;
    f>>n;
    f>>x>>y;
    a=x;b=y;
    for (i=2;i<=n;i++)
    {
        f>>x2>>y2;
        arie=arie+x*y2-y*x2;
        x=x2;
        y=y2;
    }
    arie=arie+x*a-b*y;
    arie=arie/2;
    g<<fixed<<setprecision(5)<<fabs(arie);
}