#include <fstream>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");
struct ar{
int x,y;
}a[100005];
int n;
int aria()
{long long s=0;
    for ( long long i = 1 ; i <= n ; i ++ )
        s+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    s/=2;
    return s;
}
int main()
{
    f >> n ;
     for ( long long i = 1 ; i <= n ; i ++ )
       f >> a[i].x >> a[i].y;
    a[n+1]=a[1];
   g << aria();
    return 0;
}