#include <fstream>
#include <cmath>
#include <iomanip>
#define NMAX 100001

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct punct
{
    double x,y;

}V[NMAX],Center;

int N;
double A;

double det(punct C, punct A, punct B)
{
    return C.x*A.y+A.x*B.y+B.x*C.y-B.x*A.y-A.x*C.y-C.x*B.y;
}
int main()
{
    f>>N;
    int i;
    for(i=1;i<=N;i++)
        f>>V[i].x>>V[i].y;

    for(i=1;i<N;i++)
        A+=det(Center,V[i],V[i+1]);
    A+=det(Center,V[N],V[1]);
    A/=2;

    A=abs(A);
    g<<fixed<<setprecision(6)<<A;
}