#include <fstream>
#include <cmath>
using namespace std;
ifstream cin("arie.in");
ofstream cout("arie.out");
int n;
struct punct{
    double x,y;
}v[100005];
double sol;
void read()
{
    cin>>n;
    for(int i=1;i<=n;i++)
        cin>>v[i].x>>v[i].y;
}
void solve()
{
    v[n+1]=v[1];
    for(int i=1;i<=n+1;i++)
        sol+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    cout<<fabs(sol/2.0);
}
int main()
{
    read();
    solve();
    return 0;
}