#include <fstream>
#include <vector>
#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

typedef double ld;

int n;
vector< pair<ld,ld> >v;

int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        ld x,y;
        fin>>x>>y;
        v.push_back(make_pair(x,y));
    }
    v.push_back(v[0]);
    ld arie=0;
    for(int i=1;i<=n;i++)
    {
        arie+=(double)(v[i].first-v[i-1].first)*(v[i].second+v[i-1].second);
    }
    fout<<fixed<<setprecision(6)<<fabs(0.5*arie);
    return 0;
}