#include <fstream>
#include <iomanip>
#define NMax 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct Point
{
double x,y;
};
Point P[NMax];
int N;
double Sol;
void Read()
{
    fin>>N;
    for(int i=1;i<=N;i++)
        fin>>P[i].x>>P[i].y;
    P[N+1]=P[1];
}
void Solve()
{
    for(int i=1;i<=N;i++)
        Sol+=P[i].x*P[i+1].y-P[i].y*P[i+1].x;
    //if (Sol<0) Sol*=-1;
    Sol/=2;
}
void Print()
{
    fout<<fixed<<setprecision(6)<<Sol<<'\n';
}
int main()
{
    Read();
    Solve();
    Print();
    return 0;
}