#include <bits/stdc++.h>
#define point pair<double, double>
#define x first
#define y second

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n; double x, y;
vector<point> v;

void Read()
{
  f>>n;
  for(int i = 1;i <= n;++i)
    f>>x>>y, v.push_back(make_pair(x, y));
}

void Solve()
{
  v.push_back(v[0]);
  double s = 0;
  for(int i = 0;i < v.size() - 1;++i)
    s += v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
  if(s < 0) s *= -1;
  g<<setprecision(5)<<fixed<<s / 2;
}

int main()
{
  Read();
  Solve();
  return 0;
}