#include <iostream>
#include <fstream>

using namespace std;

int main()
{
    ifstream fin ("aria.in");
    ofstream fout ("aria.out");
    int n,x,y,xant,yant,arie=0,xini,yini;
    fin>>n>>x>>y;
    xant=xini=x;
    yant=yini=y;
    for (int i=2; i<=n; i++)
    {
        fin>>x>>y;
        arie+=(xant*y-x*yant)/2;
        xant=x;
        yant=y;
    }
    arie+=(xant*yini-xini*yant)/2;
    fout<<arie;
    return 0;
}