#include <fstream>
#include <cmath>
#include <iomanip>


using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

struct ar
{
    double x,y;
} a[100003];

long double sum;

int main()
{
    int n,i;
    fin>>n;

    for(i=1; i<=n; i++)
        fin>>a[i].x>>a[i].y;
    a[n+1]=a[1];

    for(i=1; i<=n; i++)
        sum+=(a[i].x*a[i+1].y-a[i].y*a[i+1].x);
    fout<<setprecision(5)<<fixed<<sum/2.0;

    fin.close();
    fout.close();
    return 0;
}