#include <fstream>
#include <iomanip>

#define MAX_N 100000

using namespace std;

struct pct {
    long double x, y;
};

int z[5];
struct pct p[MAX_N];

long double det( struct pct a, struct pct b, struct pct c ) {
    return (b.x - a.x) * (c.y - a.y) - (c.x - a.x) * (b.y - a.y);
}

int main() {
    int n, i;
    long double arie;
    struct pct o;

    ifstream fin( "aria.in" );
    fin >> n;
    for ( i = 0; i < n; i++ )
        fin >> p[i].x >> p[i].y;

    o.x = o.y = 0;

    arie = 0;
    for ( i = 0; i < n; i++ )
        arie += det( o, p[i], p[(i + 1) % n] );
    arie /= 2;
    arie = arie > 0 ? arie : -arie;

    ofstream fout( "aria.out" );
    fout << setprecision( 5 ) << fixed << arie;

    return 0;
}