#include <fstream>
#include <iomanip>

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

const int NMax = 100005;
int n;
float arie;
pair <float , float> a[NMax];

void Solve()
{
    for(int i = 0; i < n; i++)
    {
        arie += a[i].first*a[i+1].second - a[i+1].first*a[i].second;
    }
    arie /= 2;
}

void Print()
{
    g<<fixed<<setprecision(6)<<arie;
}

void Read()
{
    f>>n;
    for(int i = 0; i < n; i++)
    {
        f>>a[i].first>>a[i].second;
    }
    a[n] = a[0];
}

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