#include <fstream>
#include <cmath>
#include <vector>
#include <iomanip>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct point
{
    long double x,y;
};
vector <point> v;
int main()
{
    int n, i;
    long double x, y;
    point p;
    fin>>n;
    for(i=1;i<=n;i++)
    {
       fin>>x>>y;
       p.x = x;
       p.y = y;
       v.push_back(p);
    }
    v.push_back(v[0]);
    long double aria = 0.0;
    for(i=0; i < n; i++)
    {
       aria = aria + ((v[i].x * v[i+1].y) - (v[i+1].x * v[i].y));
    }
    aria = fabs(aria / 2);
    fout<<fixed<<showpoint;
    fout<<setprecision(7);
    fout<<aria<<"\n";
    return 0;
}