#include <fstream>
#include <iomanip>
#include <cmath>
#define InFile  "aria.in"
#define OutFile "aria.out"
#define MAX 100001

using namespace std;

ifstream fin  (InFile);
ofstream fout (OutFile);

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

unsigned int i;

double area;

int main ()
{
    fin >> N;
    for (i=0; i<N; i++)
        fin >> x[i] >> y[i];
    x[N] = x[0];
    y[N] = y[0];
    for (i=0; i<N; i++)
        area += (y[i]+y[i+1]) * (x[i+1]-x[i]);
    area /= 2;
    fout << fixed << setprecision(5) <<abs(area);
    return 0;
}