#include <iomanip>
#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n,x[100001],y[100001];
int main()
{
    f>>n;
    for(int i=1;i<=n;++i)
        f>>x[i]>>y[i];
    double s=0; x[0] = x[n];y[0] = y[n];
    for(int i=0;i<n;++i)
        s+=(x[i]*y[i+1]-x[i+1]*y[i]);
    s/=2;
    if (s<0) s = -s;
    g<<fixed<<setprecision(6)<<s;
    return 0;
}