#include <fstream>
#include <cmath>
#include <iomanip>
#define x first
#define y second

using namespace std;

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

int n;
double sol;
pair < double, double > V[100010];

int main()
{
    in >> n;
    for (int i = 1; i <= n; ++i)
    {
        in >> V[i].x >> V[i].y;
    }

    V[n + 1] = V[1];
    for (int i = 1; i <= n; i++)
    {
        sol += (V[i].x * V[i + 1].y - V[i].y * V[i + 1].x);
    }

    out << fixed << setprecision(5) << abs(sol / 2) << '\n';

    out.close();
    return 0;
}