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

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

double calculArea(double m, double n, double p, double r)
  { return (m*r-n*p)/2;  }

int main()
{
    int n, i;
    long double AREA=0;
    fin>>n;

    double x[n];
    double y[n];

    for (i=1; i<=n; i++)
        fin>>x[i]>>y[i];

    for (i=1; i<n; i++)
        AREA+=calculArea(x[i], y[i], x[i+1], y[i+1]);

    AREA+=calculArea(x[n], y[n], x[1], y[1]);
/*
    long AA;
    int nc=0;
    AA=AREA;

    while(AA)
    {   AA=AA/10;
        nc++;  }
*/
    fout<<fixed<<setprecision(6)<<AREA;

    return 0;
}