#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,s,x[1000],y[1000],i,a;
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    for(i=1;i<=n;i++)
    {
        if(i==n)
            a=x[i]*y[1]-x[1]*y[i];
        else
        a=x[i]*y[i+1]-x[i+1]*y[i];
        a=a/2;
        s=s+a;
    }
    fout<<s;
    return 0;
}