#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

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

const int NMAX = 1e5+5;

struct point
{
    long double x, y;
};

point v[NMAX];

int main()
{
    int n, i;
    long double x, y;
    fin>>n;
    for(i=1; i<=n; i++)
    {
        fin>>x>>y;
        v[i].x = x;
        v[i].y = y;
    }

    long double a=0;

    v[n+1] = v[1];
    for(i=1; i<=n; i++)
    {
        a += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);
    }

    a /= 2.0;

    fout<<fixed<<setprecision(5)<<a;

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