#include <fstream>
#include <complex>
#define tip long double
#define read_point(U) f>>x>>y;U=complex<tip>(x,y)
#define a(U,V) imag(V*conj(U))
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
complex<tip> Z,W,S;
tip A,x,y;
int n;
int main()
{

    f>>n;
    read_point(Z);
    S=Z;
    for(int i=1;i<n;i++)
    {
        f>>x>>y;
        read_point(W);
        A+=a(Z,W);
        Z=W;
    }
    A+=a(W,S);
    g<<A;
    return 0;
}