#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
double sumar;
void aria(double x1,double y1,double x2,double y2)
{
    double a=x1*y2,b=x2*y1;
    a=a-b;
    sumar+=a;
}
int main()
{
    int n,a1,b1,ap,bp,an,bn,i;
    fin>>n;
    fin>>a1>>b1;
    ap=a1;bp=b1;
    for(i=2;i<=n;i++)
    {
        fin>>an>>bn;
        aria(ap,bp,an,bn);
        ap=an;bp=bn;
    }
    aria(an,bn,a1,b1);
    fout<<sumar/2;
    return 0;
}