#include <fstream>
#include <iomanip>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n,i;
double aria=0,x[100005],y[100005];

int main()
{
    fin>>n;
    fin>>x[1]>>y[1];
    for (i=2; i<=n; i++)
        fin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for (i=1; i<=n; i++)
        aria=aria+x[i]*y[i+1]-x[i+1]*y[i];
    fout<<fixed<<setprecision(6)<<aria/2;
    return 0;
}