#include <fstream>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n,x[100005],y[100005];
inline long long T(int i)
{long long rez;
    rez=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    return rez;
}
inline int Arie(int x[100005], int y[100005], int n)
{long long arie;
 int i;
     arie=0;
     for (i=1;i<=n;i++) arie+=T(i);
     arie+=T(1);
     return arie;
}
int main()
{int i;
    cin>>n;
    for (i=1;i<=n;i++)
     cin>>x[i]>>y[i];
    cout<<Arie(x,y,n);
    cin.close();
    cout.close();
    return 0;
}