#include <fstream>
using namespace std;
ofstream fout ("aria.out");
ifstream fin ("aria.in");
struct cel
{
    double x,y;
} v[100010];
double aria;
int n;
int main()
{
    fin>>n;
    for( int i = 1 ; i <= n ; i++ ) fin>>v[ i ].x>>v[ i ].y;
    v[ n + 1 ] = v[ 1 ];
    for( int i = 1 ; i <= n ; i++ ) aria += ( v[ i ].x * v[ i + 1 ].y - v[ i + 1 ].x * v[ i ].y );
    fout.precision(6);
    fout<<fixed<<max( aria / 2.0 , aria / -2.0 );
}