#include <iostream>
#include <fstream>
#include <iomanip>
#define dMAX 100000

using namespace std;

int n, t;

struct Point{
    double x, y;
} arr[dMAX + 2];

double A, z = 2;

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

int main()
{
    int i, j;
    fin >> n;
    for (i = 0; i < n; i++) {
        fin >> arr[i].x >> arr[i].y;
    }
    t = n;
    for (i = 0; i < n; i++) {
        A = A + (arr[i].x * (arr[(i + 1) % t].y) - arr[(i + 1) % t].x * arr[i].y);
    }

    fout << setprecision(6) << fixed << A / z;

    return 0;
}