#include <fstream>
#include <cstdio>
#include <iomanip>
std::ifstream f ("aria.in");
std::ofstream g ("aria.out");
std::pair<double, double> V[100005];
int n;
double A;
int main()
{
	f >> n;
	for(int i = 0 ; i < n; i++)
		f >> V[i].first >> V[i].second;
	V[n] = V[0];
	for(int i = 0; i < n; i++)
		A = A + (V[i].first * V[i+1].second - V[i+1].first * V[i].second);
	A = abs(A);
	A = A * 0.5;
	g<<std::fixed<<std::setprecision(7)<<A;
	return 0;
}