#include <fstream>
#define NMAX 100010

using namespace std;

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

int n,i;
int x[NMAX],y[NMAX];
long long s=0;

int main()
{
    in>>n;
    for(i=1;i<=n;i++)
    {
        in>>x[i]>>y[i];
    }
    for(i=1;i<n;i++)
    {
        s+=(x[i]*y[i+1]-y[i]*x[i+1])/2;
    }
    s+=(x[n]*y[1]-y[n]*x[1])/2;
    out<<s;
    in.close();
    out.close();
    return 0;
}