#include<fstream>
#include<iomanip>

using namespace std;

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

int N;
double Sol;

struct Punct
{
    double a,b;
};

Punct P[100005];

void Read()
{
    fin>>N;

    for(int i=1;i<=N;i++)
    {
        fin>>P[i].a>>P[i].b;
    }
}

void Solve()
{
    P[N+1]=P[1];

    for(int i=1;i<=N;i++)
    {
        Sol+=P[i].a*P[i+1].b-P[i+1].a*P[i].b;
    }

    fout<<fixed<<setprecision(6)<<Sol/2;
}

int main()
{
    Read();
    Solve();

    return 0;
}