#include <iostream>
#include <fstream>
using namespace std;

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

int calculArea(int m, int n, int p, int r)
    {
     return (m*r-n*p)/2;
    }

int main()
{
    int n, i, AREA=0;
    fin>>n;

    int x[n];
    int y[n];

    for (i=1; i<=n; i++)
        fin>>x[i]>>y[i];

    for (i=1; i<n; i++)
       {AREA+=calculArea(x[i], y[i], x[i+1], y[i+1]);
 //      cout<<AREA<<endl;
       }
        AREA+=calculArea(x[n], y[n], x[1], y[1]);
    fout<<AREA;

    return 0;
}