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

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

long double calculArea(long double m, long double n, long double p, long double r)
    {   long double triunghi=0;
        triunghi = (m*r-n*p)/2;
        if ( (m*r-n*p)/(sqrt( pow(m,2.0)+pow(n,2.0)) * sqrt(pow(p,2.0)+pow(r,2.0)) > 0))
     return (m*r-n*p)/2;
      else
     return -(m*r-n*p)/2;
    }

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

    long double x[n];
    long 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]);
 //      cout<<AREA<<endl;
       }
        AREA+=calculArea(x[n], y[n], x[1], y[1]);
    fout<<AREA;

    return 0;
}