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

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

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

long double determinant(punct a, punct b, punct c)
{
    return a.x*b.y+b.x*c.y+c.x*a.y-c.x*b.y-a.x*c.y-b.x*a.y;
}

int main()
{
    int n,i;
    long double arie=0;
    f>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
    {
        arie=arie+determinant({0,0},v[i],v[i+1]);
    }
    g<<fixed<<setprecision(5)<<max(arie,-arie)/2;
    return 0;
}