#include <fstream>
#include <cmath>
#include <iomanip>
#define sz 100000
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int n;

struct poz{
double x;
double y;}v[sz + 5];

double sol;


int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;i++)
        sol +=(v[i].x * v[i+1].y - v[i].y * v[i+1].x);
    fout<<fixed<<setprecision(6)<<fabs(sol/2.0);
}