#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
const int MAX = 100005;

struct punct{
    int x, y;
}vec[MAX];
int main()
{
    int n;
    double aria = 0;
    fin >> n;
    for (int i = 0; i < n; i++)
    {
        fin >> vec[i].x >> vec[i].y;
    }
    vec[n] = vec[0];
    for (int i = 0; i < n; i++)
    {
        aria += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
    }
    aria /= 2.0;
    fout << aria;
    return 0;
}