#include<fstream>
#include<iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");

struct punct{
    double x;
    double y;
};struct punct v[100009];
int n,x1,y1;

int main()
{

    in>>n;
    int i;
    for(i = 1 ; i <= n; i++)
        in>>v[i].x>>v[i].y;
    v[n+1].x = v[1].x;
    v[n+1].y = v[1].y;

    long double sol = 0;

    for(i = 1 ; i <= n ; i++)
        sol+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    sol/=2.0;
    out<<setprecision(6)<<sol;
    return 0;
}