#include <bits/stdc++.h>

#define x first
#define y second

using namespace std;

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

const int MAX_N = 1e5 + 5;

int n;

pair <long double, long double> points[MAX_N];

long double f(pair <long double, long double> a, pair <long double, long double> b) {
  return a.x * b.y - b.x * a.y;
}

int main() {
  long double ans;
  fin >> n;
  for (int i = 1; i <= n; ++i) {
    fin >> points[i].x >> points[i].y;
  }
  points[n + 1] = points[1];
  ans = 0.0;
  for (int i = 1; i <= n; ++i) {
    ans = (long double) (ans + abs(f(points[i], points[i + 1])));
  }
  fout << fixed << setprecision(5) << 0.5 * ans << "\n";
  return 0;
}