#include <fstream>
#include <iomanip>

using namespace std;

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

const int NMAX = 100000;

int N;
double x[NMAX+2], y[NMAX+2];

int main()
{
    double ans;
    in >> N;
    for( int i = 0; i < N; ++ i )
    {
        in >> x[ i ] >> y[ i ];
    }
    ans = 0;
    x[ N ] = x[ 0 ];
    y[ N ] = y[ 0 ];
    for( int i = 0; i < N; ++ i )
    {
        ans += ( x[ i ] * y[ i + 1 ] - x[ i + 1 ] * y[ i ] );
    }
    out << setprecision( 6 ) << fixed;
    out << 0.5 * ans << "\n";
    return 0;
}