#include <fstream>

using namespace std;
ifstream fin("arii.in");
ofstream fout("aria.out");
unsigned n,i;
int x[100050],y[100050];
float sol=0;
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
    {
        fin>>x[i]>>y[i];
    }
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
    sol =sol+ (x[i] * y[i+1] - x[i+1] * y[i])/2;
    fout<<sol;
    return 0;
}