#include <fstream>

using namespace std;

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

int n;
double x[100003],y[100003];
double sol;

int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>x[i];
        fin>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(int i=1;i<=n;i++)
    {
        sol+=x[i]*y[i+1]-x[i+1]*y[i];
    }
    sol/=2;
    fout<<max(sol,-sol);
}