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

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

struct punct
{
    double x,y;
};
long double arie_triunghi(punct A, punct B)
{
    return (A.x*B.y-A.y*B.x)/2;
}
void citire()
{
    int n;
    fin>>n;
    long double arie=0;
    punct primul, anterior;
    fin>>primul.x>>primul.y;
    anterior=primul;
    for(int i=1;i<n;i++)
    {
        punct X;
        fin>>X.x>>X.y;
        arie+=arie_triunghi(X,anterior);
        anterior=X;
    }
    arie+=arie_triunghi(primul, anterior);
    fout<<fixed<<setprecision(6)<<abs(arie);
}
int main()
{
    citire();
    return 0;
}