#include <fstream>
#include <vector>
using namespace std;

ifstream is("aria.in");
ofstream os("aria.out");

struct Punct{
    double x, y;
} in, a, b;

int n;
double s;

int Aria(Punct A, Punct B);

int main()
{
    is >> n;
    is >> a.x >> a.y;
    in.x = a.x; in.y = a.y;
    for ( int i = 2; i <= n; ++i )
    {
        is >> b.x >> b.y;
        s += Aria(a, b);
        a = b;
    }
    s += Aria(a, in);
    os << s;
    is.close();
    os.close();
    return 0;
}

int Aria(Punct A, Punct B)
{
    return ( A.x * B.y - A.y * B.x ) / 2;
}