#include <fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
int main()
{
    int n,a1,a2,b1,b2,c1,c2,delta,i,s=0;
    in>>n>>a1>>a2>>b1>>b2;
    for(i=3;i<=n;i++)
    {
        in>>c1>>c2;
        delta=a1*b2+b1*c2+c1*a2-b2*c1-c2*a1-a2*b1;
        s+=delta/2;
        a1=b1;
        b1=c1;
        a2=b2;
        b2=c2;
    }
    out<<s;
    return 0;
}