#include <fstream>


using namespace std;

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    int A=0,n,x[100005],y[100005],i;
    fin>>n;
    for (i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[i]=x[1];
    y[i]=y[1];
    for (i=1;i<=n;i++)
        A+=x[i]*y[i+1]-x[i+1]*y[i];
    A=A/2;
    fout<<A;
    return 0;
}