#include <fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
pair <float,float> v[10001];
int main()
{
    int n,i,delta,s=0;
    in>>n;
    for(i=1;i<=n;i++)
    in>>v[i].first>>v[i].second;
    for(i=1;i<=n-2;i++)
    {
        delta=(v[i].first*v[i+1].second+v[i+1].first*v[i+2].second+v[i+2].first*v[i].second-v[i].second*v[i+1].first-v[i+1].second*v[i+2].first-v[i+2].second*v[i].first);
        s+=delta;
    }
    out<<s;
    return 0;
}