#include <iostream>
#include <fstream>
#include <vector>
#include <cmath>
#define nmax 100010
using namespace std;

int n;
double sol;
vector <pair <double, double> > v(nmax);

void read()
{
    ifstream f("aria.in");
    f >> n;
    for(int i=0; i<n; ++i)
        f >> v[i].first >> v[i].second;
    f.close();
}

void out()
{
    ofstream g("aria.out");
    v[n] = v[0];
    for(int i=0; i<n; ++i)
    sol += (v[i].first * v[i+1].second - v[i].second * v[i+1].first);
    g << fabs(sol/2.0);
    g.close();
}

int main()
{
    read();
    out();
    return 0;
}