#include<fstream>
#include<iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
long double N,x1,x2,y1,y2,X,Y,S;
int main()
{
    f>>N>>x1>>y1,X=x1,Y=y1;
    for(int i=2;i<=N;++i){
        f>>x2>>y2;
        S+=x1*y2-x2*y1;
        x1=x2,y1=y2;
    }
    S+=(x1*Y-X*y1),S/=2;
    g<<S<<setprecision(5);
    return 0;
}