#include <iostream>
#include <fstream>
#define NMAX 100000

using namespace std;

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

int n;
double area;
pair <double, double> v[NMAX+10];

int main()
{
    f >> n;
    for(int i=1; i<=n; i++) f >> v[i].first >> v[i].second;
    for(int i=2; i<=n; i++) area = area + (v[i-1].first * v[i].second - v[i-1].second * v[i].first);
    area = area + (v[n].first * v[1].second - v[1].first * v[n].second);
    area /= 2;
    g << area << '\n';
    return 0;
}