#include <fstream>
#include<iomanip>
using namespace std;

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

long double s=0;
struct punct
{
    double x,y;
}v[100010];

int i,n;

int main()
{
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    for(i=2;i<=n;i++)
        s+=v[i-1].x*v[i].y-v[i].x*v[i-1].y;
    s+=v[n].x*v[1].y-v[1].x*v[n].y;
    g<<setprecision(5)<<fixed<<s/2;
    return 0;
}