#include <cstdio>
#include <cmath>

const int MAX = 100005;

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

int N;
double sol;

int main (void) {
  freopen ("aria.in", "r", stdin);
  freopen ("aria.out", "w", stdout);
  scanf ("%d", &N);
  for (int i = 0; i < N; ++i)
    scanf ("%lf %lf", &vec[i].x, &vec[i].y) == 2;
  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));
}