#include <bits/stdc++.h>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
// #define f cin
// #define g cout
// code-runner.runInTerminal
struct point
{
    double x, y;
    friend istream &operator>>(istream &is, point &t)
    {
        is >> t.x >> t.y;
        return is;
    }
    point operator-(const point &t) const
    {
        point rez = {x - t.x, y - t.y};
        return rez;
    }
    double operator*(const point &t) const
    {
        double ar = (double)x * t.y - (double)y * t.x;
        return ar;
    }
    double arrel(const point &a, const point &b) const
    {
        return (a - *this) * (b - *this);
    }
};
int main()
{
    int n;
    f >> n;
    vector<point> v(n);
    for (auto &i : v)
        f >> i;
    double rez = 0;
    for (size_t i = 0; i < v.size(); i++)
    {
        size_t j = (i + 1) % v.size();
        rez += v[i] * v[j];
    }
    rez /= 2.0;
    g << fixed << setprecision(6) << rez;
    return 0;
}