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

#define xy std::pair<float,float>

using namespace std;

vector<xy> coord;
int n;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    xy per,per1;
    float res=0;
    fin>>n;
    fin>>per.first>>per.second;
    coord.push_back(make_pair(per.first,per.second));
    for(int i=0;i<n-1;i++)
    {
        fin>>per1.first>>per1.second;
        coord.push_back(make_pair(per1.first,per1.second));
    }
    coord.push_back(make_pair(per.first,per.second));
    for(int i=0;i<n;i++)
    {
        res+=((coord[i].second+coord[i+1].second)/2)*(coord[i+1].first-coord[i].first);
    }
    fout.precision(5);
    fout<<fixed<<fabs(res);
    return 0;
}