#include <iostream>
#include <fstream>

using namespace std;

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

const int DIM =1e5 + 7;

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

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

    for(int i = 1;i <= n;i++)
       in >> v[i].x >> v[i].y;

       double rez = 0;
       v[n + 1] = v[1];
    for(int i = 1; i <= n; i++)
        rez += ((v[i].x * v[i+1].y) - (v[i+1].x * v[i].y));
    out << rez / 2;
    return 0;
}