#include <fstream>
#include <iomanip>
#include<cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
int main()
{
    f>>n;
    double a,b;
    f>>a>>b;
    double k1=a,k2=b;
    long double rez=0;
    double x,y;
    for(int i=1;i<n;i++)
    {
        f>>x>>y;
        rez+=0.5*abs(1.0*a*y-b*x*1.0);
        a=x;b=y;
    }
    a=k1;
    b=k2;
    rez+=0.5*abs(1.0*a*y-b*x*1.0);
    g<<setprecision(10000)<<rez;

}