#include <iostream>
#include <iomanip>
#include <fstream>
#include <cmath>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct punct
{
    long double x, y;
};

int n;
long double A, T;
punct v[100010];

int main()
{
    f >> n;
    for(int i = 1; i <= n; i++)
        f >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    for(int i = 1; i <= n; i++)
    {
        A = v[i].x * v[i + 1].y - v[i].y * v[i + 1].x;
        T += A;
    }
    if(T < 0) T = -T;
    g << fixed << setprecision(5) << T / 2.0;
    f.close();
    g.close();
    return 0;
}