#include <iostream>
#include <iomanip>
#include <fstream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

struct Punct
{
    double x;
    double y;
};

Punct P[100005];
int N,Sol;

void Read()
{
    fin>>N;
    for(int i=1;i<=N;++i)
        fin>>P[i].x>>P[i].y;
}

void Aria()
{
    for(int i=1;i<=N;++i)
        Sol+=P[i].x*P[i+1].y-P[i+1].x*P[i].y;
    Sol/=2;
}
void Print()
{
    fout<<fixed<<setprecision(6)<<Sol<<"\n";
}
int main()
{
    Read(); Aria(); Print(); return 0;
}