#include <fstream>
#include <iomanip>
#define db long double
using namespace std;
db x[100005],y[100005],a;
int main()
{
    ifstream fi("aria.in");
    ofstream fo("aria.out");
    int n;fi>>n;
    for (int i=1;i<=n;fi>>x[i]>>y[i],++i);x[0]=x[n];y[0]=y[n];
    for (int i=0;i<n;++i) a+=x[i]*y[i+1]-x[i+1]*y[i];a=a/2;
    fo<<fixed<<setprecision(8)<<a<<"\n";
}