#include <fstream>
#include <iomanip>
#include <cmath>
#define DM 100000

using namespace std;

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

double x[DM], y[DM];
unsigned int N;

unsigned int i;

double area;

void read ();
void solve ();
void print ();

int main ()
{
    read ();
    solve ();
    print ();
    return 0;
}

void read ()
{
    fin >> N;
    for (i=0; i<N; i++)
        fin >> x[i] >> y[i];
}

void solve ()
{
    x[N+1] = x[1];
    y[N+1] = y[1];
    area = 0;
    for (i=0; i<=N; i++)
        area += (y[i] + y[i+1]) * (x[i+1] - x[i]);
    area /= 2;
}

void print ()
{
    fout << fixed << setprecision (5) << abs (area);
}