#include <fstream>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int n,x0,y0,x,y,xi,yi,i;
    int64_t s=0;
    fin>>n>>x0>>y0;
    xi=x0;
    yi=y0;
    for(i=1;i<n;++i)
    {
        fin>>x>>y;
        s=s+(xi*y-x*yi)/2;
        xi=x;
        yi=y;
    }
    s=s+(xi*y0-x0*yi)/2;
    fout<<s;
}