#include <iomanip>
#include <fstream>
using namespace std;
ifstream f( "aria.in" );
ofstream g( "aria.out") ;
int n;
double a[100001], b[100001], ans;
int main()
{
   f>>n;
    for( int i= 0; i< n; ++i )
    f>>a[i]>>b[i];
    a[n]=a[0];
    b[n]=b[0];
    for( int i= 0; i< n; ++i )
    ans += (a[i]*b[i+1]-a[i+1]*b[i]);

    g << setprecision(6) << fixed;
    g << 0.5 * ans;

    return 0;
}