#include <cstdio>

const int MAXN = 1e6;

struct pol {
  int x, y;
} v[MAXN + 1];

int main() {
  int n;
  double ar;
  FILE *f = fopen("aria.in", "r");
  fscanf(f, "%d", &n);
  for (int i = 0; i < n; ++i) {
    fscanf(f, "%d%d", &v[i].x, &v[i].y);
  }
  fclose(f);
  ar = 0;
  v[n] = v[0];
  for (int i = 0; i < n; ++i) {
    ar += v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
  }
  f = fopen("aria.out", "w");
  fprintf(f, "%.5lf\n", ar / 2.0);
  fclose(f);
  return 0;
}