#include <fstream>
#include <cmath>
#include <iomanip>
#define sz 100000
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int n;

struct poz{
long double x;
long double y;}v[sz + 5];

long double sol;


int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    sol=0;
    for(int i=1;i<=n;i++)
        sol +=v[i].x * v[i+1].y - v[i+1].x * v[i].y;
    sol/=2;
    if(sol<0)
        sol=-sol;
    fout<<fixed<<setprecision(5)<<sol;
}