#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
long long a,i,n;
struct punct
{
    int x,y;
}v[100100];
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    for(i=2;i<n;i++)
    {
        a+= v[1].x * v[i].y + v[i].x * v[i+1].y + v[i+1].x *v[1].y - v[i+1].x * v[i].y - v[i].x * v[1].y - v[1].x * v[i+1].y;
    }
    fout<<a/2;
    return 0;
}