#include <fstream>
#include <vector>

using namespace std;

typedef long long ll;

double det(pair<double, double> a, pair<double, double> b)
{
    return a.first*b.second - a.second*b.first;
}
int n;
double aria;
vector<pair<double, double> >v;

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    double x, y;
    fin>>n;
    for(int i=1; i<=n; i++)
    {
        fin>>x>>y;
        v.push_back({x, y});
    }
    v.push_back(v[0]);
    for(int i=0; i<v.size()-1; i++)
        aria += det(v[i], v[i+1]);
    aria /= 2;
    fout.precision(6);
    fout<<fixed<<aria<<'\n';
    return 0;
}