# include <cstdio>
# include <cmath>
const int MAX = 100005;
struct punct
{
    double x, y;
} v[MAX];
int N;
double sol;

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

    scanf ("%d", &N);
    for (int i = 0; i < N; ++i)
        scanf ("%lf %lf", &v[i].x, &v[i].y);

    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));
}