#include <fstream>
#define x first
#define y second
#include <iomanip>

using namespace std;
ifstream in ( "aria.in" );
ofstream out( "aria.out" );
pair < long double, long double > v[1000001];
long double sol;
int n, i;

long double arie ( pair < long double, long double > a, pair < long double, long double > b, pair < long double, long double > c ){
    return (b.x-a.x) * (c.y-a.y) - (c.x-a.x)*(b.y-a.y);
}

int main() {
    in>>n;
    for ( i=1; i <= n; i++ ) in >> v[i].x >> v[i].y;
    for ( i=1; i < n; i++ )
        sol += arie ( v[n], v[i], v[i+1] );
    out << setprecision(7) << fixed << sol/2;
    return 0;
}