#include <bits/stdc++.h>

using namespace std;

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

const int MAXN = 1e5;

long double x[MAXN + 1], y[MAXN + 1];

int main()
{
  int n, i;
  long double sol;
  fin >> n;
  for (i = 1; i <= n; i++)
    fin >> x[i] >> y[i];
  x[n + 1] = x[1];
  y[n + 1] = y[1];
  sol = 0;
  for (i = 1; i <= n; i++)
    sol = sol + x[i] * y[i + 1] - x[i + 1] * y[i];
  sol = sol / 2;
  fout << setprecision(6) << fixed << sol;
}