#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
const int nmax=1e5+3;
long double sol;
int n;
struct
{
    long double x,y;
}v[nmax];
int main()
{
    f>>n;
    for(int i=1;i<=n;++i) f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(int i=1;i<=n;++i) sol=sol+v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    sol=sol/2;
    if(sol<0) sol=-sol;
    g<<fixed<<setprecision(6)<<sol;
    return 0;
}