#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
const int NMAX=100005;
struct pct
{
    double x;
    double y;
}v[NMAX];
double arie(int i)
{
    return (v[i].x * v[i+1].y-v[i+1].x * v[i].y)/2;
}
int main()
{
    ifstream si;
    si.open("aria.in");

    ofstream so;
    so.open("aria.out");
    int n;
    si>>n;
    int i;
    for(i=0;i<n;++i)
    {
        si>>v[i].x>>v[i].y;
    }
    v[n]=v[0];
    double sum=0;
    for(i=0;i<n;++i)
        sum+=arie(i);
    so<<sum<<'\n';
}