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

using namespace std;

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

struct vector2{
double x,y;
};

double arie(vector2 a,vector2 b,vector2 c)
{
    return a.x * b.y + a.y * c.x + b.x * c.y - a.x * c.y - a.y * b.x - b.y * c.x;
}

int n;
double A;

vector2 Or;
vector2 lst,act;

int main()
{
    fin>>n;

    fin>>Or.x >>Or.y;
    fin>>lst.x>>lst.y;

    for(int i=1;i<=n;i++)
    {
        fin>>act.x>>act.y;

        A+= arie(Or,lst,act);

        lst = act;
    }

    fout<<std::setprecision(6);
    fout<<std::fixed;
    fout<<abs(A) / 2<<'\n';

    return 0;
}