#include <iomanip>
#include <fstream>
#define x first
#define y second
using namespace std;

int n, i, a, b;

double aria;

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

pair < double, double > v[100005];

int main ()
{
    f>>n;

    for ( i = 1 ; i <= n; i++ )
    {
        f>>v[i].x>>v[i].y;
    }

    v[n+1]=v[1];

    for ( i = 1 ; i <= n ; i ++ )
    {
        aria += ( ( v[i].x * v[i+1].y - v[i+1].x * v[i].y ) / 2 );
    }

    if ( aria < 0 )
    {
        aria = -aria;
    }

    g<<setprecision(5)<<fixed<<aria;

    return 0;
}