#include <fstream>
#include <iomanip>
using namespace std;

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

const int NMax = 100000;
pair<double, double> P[NMax + 5];
int N;
double Sol;

void Read()
{
    fin>>N;
    for(int i = 1; i<=N; i++)
            fin>>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+1].first*P[i].second;
    Sol = Sol / 2;
}

void Print()
{
    fout<<fixed<<setprecision(6)<<Sol<<"\n";
}

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