#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

double det(pair < int, int > a, pair < int, int > b, pair < int, int > c) {
  double d = (a.first * b.second) + (b.first * c.second) + (c.first * a.second) -
          (a.second * b.first) - (b.second * c.first) - (c.second * a.first);

  return d / 2.0f;
}

int main() {
  int n;
  cin >> n;
  vector < pair < int, int > > v(n + 1);

  double aria = 0;
  pair < int, int > p = {100, 100};
  for (int i = 0; i < n ; i ++) {
    int x, y;
    cin >> x >> y;
    v[i] = {x, y};
  }

  for (int i = 0; i < n; i++) {
    aria += det(v[i], v[(i + 1)% n], p); 
  }

  cout << aria << '\n';
}