#include <fstream>
#include <algorithm>
#include <vector>
#include <cstring>
#include <map>
#include <iomanip>
#include <set>
#define x first
#define y second
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,n;
double sol,a,b;
vector< pair<double,double> > v;
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>a>>b;
        v.push_back(make_pair(a,b));
    }
    v.push_back(v[0]);

    for(i=0;i<n;i++)
        sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2.0;
    if(sol<0)
        sol*=-1.0;
    g<<setprecision(6)<<sol;
    return 0;
}