#include<fstream>
#include<iomanip>
using namespace std;
ifstream F("aria.in");
ofstream G("aria.out");
int n,i;
long double x[100001],y[100001],s;
int main()
{
    F>>n;
    for(i=0;i<n;++i)
        F>>x[i]>>y[i];
    for(x[n]=x[0],y[n]=y[0],i=0;i<n;++i)
        s=s+x[i]*y[i+1]-x[i+1]*y[i];
    G<<setprecision(5)<<fixed<<s/2;
    return 0;
}