#include <fstream>
#include <iomanip>

const int nmax=100001;

using namespace std;

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");

    int n,i;
    long double x[nmax],y[nmax];

    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];

    x[n+1]=x[1];
    y[n+1]=y[1];

    long double s=0;
    for(i=1;i<=n;i++) {
        s+=(x[i]*y[i+1]-x[i+1]*y[i]);
    }
    if(s<0)
        s=-s;
    s/=2;

    fout<<setprecision(6)<<s<<fixed;

    fin.close();
    fout.close();

    return 0;
}