# include <cassert>
# include <cstdio>
# include <cmath>
#define MAX 100005


struct punct
{
    double x, y;
} v[MAX];

int N;
double sol;

int main (void)
{
    assert (freopen ("aria.in", "r", stdin));
    assert (freopen ("aria.out", "w", stdout));

    assert (scanf ("%d", &N) == 1);
    assert (1 <= N && N <= 100000);
    for (int i = 0; i < N; ++i) {
        assert (scanf ("%lf %lf", &v[i].x, &v[i].y) == 2);
        assert (-10000000 <= v[i].x && v[i].x <= 10000000);
        assert (-10000000 <= v[i].y && v[i].y <= 10000000);
    }
    v[N] = v[0];
    for (int i = 0; i < N; ++i)
        sol += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);
    printf ("%lf", fabs (sol / 2.0));
}