#include<fstream>
#include<iomanip>
#define LM 100004
using namespace std;

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

struct Pct
{
    double a,b;
};

Pct P[LM];

int N;
double Sol;

int main()
{
    int i;

    fin>>N;

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

    P[N+1]=P[1];

    for(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<<"\n";

    fin.close();
    fout.close();
    return 0;
}