#include <fstream>



using namespace std;

ifstream in ("aria.in");

ofstream out ("aria.out");

int v1[100002],v2[100002];



int main()

{

    int n;

    int i;

    int a=0;

    in>>n;

    for(i=1;i<=n;i++)

    in>>v1[i]>>v2[i];

    v1[n+1]=v1[1];

    v2[n+1]=v1[1];

    for(i=1;i<=n;i++)

    {

        a=a+(v1[i]*v2[i+1]-v1[i+1]*v2[i])/2;

    }

    out<<a;

    return 0;

}