#include <stdio.h>
#include <stdlib.h>
#include <math.h>
FILE *fin, *fout;

struct Punct {
    double x, y;
};
struct Punct v[100001];
int main(){
  int n;
  fin = fopen("aria.in", "r");

  fscanf(fin, "%d", &n);
  for (int i = 0;i<n;i++){
    fscanf(fin, "%lf%lf", &v[i].x, &v[i].y);
  }
  fclose(fin);
  double arie = 0;
  for (int i = 0;i<n;i++){
    int j = (i + 1) % n;
    arie += v[i].x * v[j].y - v[j].x * v[i].y;
  }
  arie /= 2;
  fout = fopen("aria.out", "w");
  fprintf(fout, "%lf", arie);
  fprintf(fout, "\n");
  fclose(fout);
  return 0;
}