#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin ( "aria.in" );
ofstream fout ( "aria.out" );
const int NMAX = 1e5;
struct p {
    double x, y;
} v[NMAX + 2];
double det ( p a, p b ) {
    return ( a.x * b.y - a.y * b.x );
}
int main () {
    int n;
    double s;
    fin >> n;
    for ( int i = 0; i < n; i++ )
        fin >> v[i].x >> v[i].y;
    s = 0;
    v[n] = v[0];
    for ( int i = 0; i < n; i++ )
        s += det ( v[i], v[i + 1] );
    fout << fixed << setprecision ( 5 ) << s / 2;
    
    return 0;
}