#include <fstream>
#include <vector>

using namespace std;

int main(void)
{
	ifstream fin("aria.in");
	ofstream fout("aria.out");
	int n;
	fin>>n;
	vector<double> vecX, vecY;
	for(int i=0; i<n; i++)
	{
		double x, y;
		fin>>x>>y;
		vecX.push_back(x);
		vecY.push_back(y);
	}

	double sum = 0.0;
	for(int i=0; i<n-1; i++)
	{
		sum += vecX[i] * vecY[i+1] - vecX[i+1] - vecY[i];
	}
	double aria = sum / 2.0;
	fout<<aria<<endl;
	return 0;
}