#include <cstdio>
#include <cmath>
using namespace std;

double abs(double x){
  if(x<0)
    return -x;
  return x;
}

int main()
{
  int n, i;
  double x1, y1, xi, yi, x2, y2, s;
  FILE *fi=fopen("aria.in", "r"), *fo=fopen("aria.out", "w");
  fscanf(fi, "%d", &n);
  fscanf(fi, "%lf%lf", &x1, &y1);
  xi=x1;
  yi=y1;
  s=0;
  for(i=0;i<n;i++){
    fscanf(fi, "%lf%lf", &x2, &y2);
    s=s+x1*y2-x2*y1;
    x1=x2;
    y1=y2;
  }
  s=s+x1*yi-xi*y1;
  s/=2.0;
  s=fabs(s);
  fprintf(fo, "%lf", s);
  fclose(fi);
  fclose(fo);
  return 0;
}