#include <bits/stdc++.h>
using namespace std;

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


struct punct
{
    long double x, y;
};

int main()
{
    int N, i, j;
    fin >> N;
    vector<punct> points(N + 2);

    for (i = 1; i <= N; i++)
    {
        fin >> points[i].x >> points[i].y;
    }
    points[N+1] = points[1];

    long double area = 0.0;
    for (i = 1; i <= N; i++)
        area += (points[i].x * points[i+1].y - points[i+1].x * points[i].y);

    area = fabs(area) / 2;
    fout  << setprecision(5)<< fixed << area;

    fin.close();
    fout.close();
    return 0;
}