#include <fstream>
#include<cmath>
#include<vector>
using namespace std;

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

int n,i;
vector<pair<double,double> > v;
double a,b,sol;
int main()
{
    fin>>n;
    for(i=1;i<=n;++i)
    {
        fin>>a>>b;
        v.push_back(make_pair(a,b));
    }
    v.push_back(v[0]);
    for(i=0;i<n;++i)
    {
        //formula aria din determinant
        sol+=v[i].first*v[i+1].second-v[i].second*v[i+1].first;
    }
    fout<<abs(sol/2.0);

    return 0;
}