#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

#define lim 100010
struct point {int x,y;} ini[lim];
int n;
double rez=0;


double arie (point a, point b, point c)
{
    return (double) a.x*b.y + c.x*a.y + b.x*c.y - c.x*b.y - b.x*a.y - a.x*c.y;
}


int main()
{
    fin>>n;
    for (int i=1; i<=n; i++)
        fin>>ini[i].x>>ini[i].y;
    ini[++n]=ini[1];

    for (int i=1; i<=n-1; i++)
        rez += ini[i].x*ini[i+1].y - ini[i].y*ini[i+1].x;
    fout<<setprecision(5)<<fixed<<(double)rez/2;

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