#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;

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/2.0;

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