#include <fstream>
#include <iomanip>
#include <cmath>

int main(){
	std::ifstream fin("aria.in");
	std::ofstream fout("aria.out");

	unsigned n; fin>>n;
	long double A=0;

	if(n>2){
		long double x0,y0,xp,yp;
		fin>>xp>>yp;
		x0=xp; y0=yp;

		long double x,y;

		for(unsigned i=1;i<n;++i){
			fin>>x>>y;
			A+=xp*y-x*yp;
			xp=x; yp=y;
		}

		A+=xp*y0-x0*yp;
	}



	fout<<std::fixed<<std::setprecision(5)<<0.5*std::abs(A)<<'\n';
}