#include<fstream>
#include<cmath>
#include<iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int i, n;
    long double s, x1, y1, x2, y2, aux1, aux2;
    fin >> n;
    fin >> x1 >> y1;
    aux1=x1; aux2=y1;
    s=0;
    for (i=2; i<=n; i++)
    {
       fin >> x2 >> y2;
       s = s + (x1*y2-x2*y1);
       x1=x2; y1=y2;
    }
    s += (x1*aux2-aux1*y1);
    s=fabs(s);
    s=s/2;
    fout<<fixed<<setprecision(5)<<s;
    return 0;
}