#include <fstream>
#define Nmax 100005
using namespace std;
ifstream f("aria.in"); ofstream g("aria.out");
struct Point{double x, y;} A[Nmax];
int n;
double S;
int main()
{
    f>>n;
    for(int i=0; i<n; i++)
    {
        f>>A[i].x>>A[i].y;
    }
    A[n]=A[0];
    for(int i=0; i<n; i++)
    {
        S+=A[i].x*A[i+1].y-A[i+1].x*A[i].y;
    }
    g<<S/2.;
    g.close(); return 0;
}