#include <fstream>
#define x first
#define y second

using namespace std;

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

typedef pair <double, double> point;
const int NMAX=100002;

point v[NMAX];
int n,sol;

void read ()
{
    fin>>n;
    for (int i=0; i<n; i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    v[n]=v[0];
}

void solve ()
{
    sol=0;
    for (int i=0; i<n; i++)
    {
        sol+=(v[i].x * v[i+1].y - v[i+1].x * v[i].y);
    }
    sol/=2;
    fout<<sol;
}

int main ()
{
    read ();
    solve ();
    fin.close (); fout.close ();
    return 0;
}