#include <cstdio>
using namespace std;

int main () {

    freopen ("aria.in", "r", stdin);
    freopen ("aria.out", "w", stdout);
    double x, _x, x1, y, _y, y1, A = 0;
    int N, i;
    scanf ("%d", &N);
    scanf ("%lf%lf", &x1, &y1);
    _x = x1;
    _y = y1;
    for (i = 2; i <= N; ++i)
        scanf ("%lf%lf", &x, &y),
        A += _x * y - _y * x,
        _x = x,
        _y = y;
    A += x * y1 - y * x1;
    printf ("%.5f", A / 2);

}