#include <iomanip>
#include <fstream>
#include <algorithm>

using namespace std;

const int NMAX = 100000;

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

int N;
double A[NMAX+1], B[NMAX+1], ans;

int main()
{
    in >> N;

    for( int i= 0; i< N; ++i )
    {
        in >> 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] );
    }

    out << setprecision( 6 ) << fixed;
    out << 0.5 * ans << '\n';

    return 0;
}