# include <cstdio>
# include <cmath>
# include <fstream>
# include <iomanip>
# define x first
# define y second
std::ifstream f ("aria.in");
std::ofstream g ("aria.out");
std::pair<double, double> vec[100005];
int N;
double sol;
int main () 
{
    f >> N;
    for (int i = 0; i < N; ++i)
        f >> vec[i].x >> vec[i].y; 
    vec[N] = vec[0];
    for (int i = 0; i < N; ++i)
        sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
     g<<std::fixed<<std::setprecision(7)<<abs(sol*0.5); //printf ("%lf", fabs (sol / 2.0));
}