#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 = 0 ;
       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);

        if(rez < 0)
            rez *= -1;
        rez /= 2.0;

    out <<fixed<<setprecision(9)<< rez;
    return 0;
}