#include <fstream>

using namespace std;
double x[1000002], y[1000002];
int i,N,A;
ifstream cin("aria.in");
ofstream cout("aria.out");
int main()
{
    cin>>N;
    for(i=1; i<=N; i++)
    {
        cin>>x[i]>>y[i];
    }
    x[N+1]=x[1];
    y[N+1]=y[1];
    for(i=1; i<=N; i++)
    {
        A=x[i]*y[i+1]-x[i+1]*y[i];
    }
    cout<<A/2;
    cin.close();
    cout.close();
    return 0;
}