# include <cassert>

# include <cstdio>

# include <cmath>



const char *FIN = "aria.in", *FOU = "aria.out";

const int MAX = 100005;



struct punct {

    double x, y;

} vec[MAX];



int N;

double sol;



int main (void) {

    assert (freopen (FIN, "r", stdin));

    assert (freopen (FOU, "w", stdout));



    assert (scanf ("%d", &N) == 1);

    assert (1 <= N && N <= 100000);

    for (int i = 0; i < N; ++i) {

        assert (scanf ("%lf %lf", &vec[i].x, &vec[i].y) == 2);

        assert (-10000000 <= vec[i].x && vec[i].x <= 10000000);

        assert (-10000000 <= vec[i].y && vec[i].y <= 10000000);

    }

    vec[N] = vec[0];

    for (int i = 0; i < N; ++i)

        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);

    printf ("%lf", fabs (sol / 2.0));

}