#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,j,n,k;
long double s=0;
struct gogu
{
    long double x,y;
}v[1000010];
long double modul(long double x)
{
    if(x<0)
        return -x;
    return x;
}
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>v[i].x>>v[i].y;
        if(i>=2)
        {
            s=s+(v[i-1].x*v[i].y-v[i].x*v[i-1].y);
        }
    }
    s=s+(v[n].x*v[1].y-v[1].x*v[n].y);
    g<<setprecision(5)<<fixed<<modul(s)/2;
}