#include <bits/stdc++.h>

using namespace std;

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

const int DIM =1e5 + 7;

struct punct
{
   long double x,y;
}v[DIM];

 main()
{
    int n;
    in >> n;

    for(int i = 0;i < n; i++)
       in >> v[i].x >> v[i].y;

       long double rez ;
       v[n] = v[0];
    for(int i = 0; i < n; ++i)
        rez += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);

    out <<fixed<<setprecision(6)<< fabs(rez / 2.0) ;
    return 0;
}