#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout("aria.out");
int n,i;
long double sol;
struct elem{
    long double x,y;
}V[100005];
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>V[i].x>>V[i].y;
    V[n+1]=V[1];
    sol=0;
    for(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;
    fout<<fixed<<setprecision(5)<<sol;
    return 0;
}