#include <fstream>
#include <bits/stdc++.h>

using namespace std;

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

struct punct {
double x, y;
};

punct plan[10005];

int n;
double sum = 0;

int main()
{
    fin >> n;

    for (int i=0; i<n; i++) fin >> plan[i].x >> plan[i].y;

    plan[n].x = plan[0].x;
    plan[n].y = plan[0].y;

    for (int i=0; i<n; i++) sum += plan[i].x * plan[i+1].y - plan[i].y * plan[i+1].x;

    fout << fixed << setprecision(5) << sum / 2;

    return 0;
}