#include <fstream>
using namespace std;
int main()
{
    ifstream in("aria.in");
    ofstream out("aria.out");
    long long aria=0, x1, y1, xi, yi, xi2, yi2;
    long n;
    in>>n;
    in>>xi>>yi;
    x1=xi; y1=yi;
    for(long i=2; i<=n; ++i)
    {
        in>>xi2>>yi2;
        aria+=xi*yi2-xi2*yi;
        xi=xi2;
        yi=yi2;
    }
    xi2=x1;
    yi2=y1;
    aria+=xi*yi2-xi2*yi;
    aria/=2;
    out<<aria<<'\n';
    in.close(); out.close();
    return 0;
}