#include <fstream>
#include <iomanip>
const int NMAX=100005;

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

typedef long double ld;

struct punct
{
  ld x, y;
}v[NMAX];

ld aria(punct [], int);

int n;

int main()
{
  int i;
  ld ans;
  fin>>n;
  for(i=1; i<=n; i++) fin>>v[i].x>>v[i].y;
  v[n+1]=v[1];
  ans=aria(v, n);
  fout<<fixed<<setprecision(6)<<ans<<'\n';
  return 0;
}

ld aria(punct v[], int n)
{
  int i;
  ld sum=0;
  for(i=1; i<=n; i++) sum+=(1.0/2.0)*(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
  return sum;
}