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

using namespace std;

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

#define nmax 100005

struct Coordonate
{
    long double x,y;
}v[nmax];

int n;

long double sol=0.0;

int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1].x=v[1].x;
    v[n+1].y=v[1].y;
    for(int i=1;i<=n;i++)
        sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    sol/=2.0;
    fout<<fixed<<setprecision(10)<<sol<<'\n';
}