#include <fstream>


using namespace std;

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    int n,i;
    double A=0.00,x[100005],y[100005];
    fin>>n;
    for (i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[i]=x[1];
    y[i]=y[1];
    for (i=1;i<=n;i++)
        A+=(double)(x[i]*y[i+1]-x[i+1]*y[i]);
    A=(double)A/2;
    fout<<A;
    return 0;
}