#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

const int Nmax = 100005;

pair <float, float> P[Nmax];
int n;
float Sol;

void Read()
{
    f>>n;
    for(int i = 1; i <= n; i++) f>>P[i].first>>P[i].second;
    P[n+1] = P[1];
}

void Solve()
{
    for(int i = 1; i <= n; i++)
        Sol = Sol + P[i].first*P[i+1].second - P[i].second*P[i+1].first;
}

void Print()
{
    g<<fixed<<setprecision(6)<<Sol/2<<'\n';
}

int main()
{
    Read();
    Solve();
    Print();
    return 0;
}