#include <bits/stdc++.h>

#define x first
#define y second

using namespace std;

const int nmax = 1e5 + 10;

int n;
pair < double, double > a[nmax];

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

    fin >> n;
    for (int i = 1; i <= n; ++i)
        fin >> a[i].x >> a[i].y;

    a[n+1] = a[1];

    double area = 0.0;
    for (int i = 1; i <= n; ++i)
        area += a[i].x * a[i+1].y - a[i].y * a[i+1].x;

    area /= 2; area = max(area, -area);

    fout << fixed << setprecision(10);
    fout << area << '\n';

    return 0;
}