// Playground.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>
#include <math.h>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

vector<pair<int, int>> cont;

int main()
{
	int n;
	fin >> n;
	for (int i = 0; i < n; i++)
	{
		int x, y;
		fin >> x >> y;
		cont.push_back({ x,y });
	}
	cont.push_back(cont.front());

	double area = 0;
	for (auto iter = cont.begin(); std::next(iter) != cont.end(); iter++)
	{
		auto nextIter = std::next(iter);
		area += ((int)(iter->first) * nextIter->second) - (int)(nextIter->first) * iter->second;
	}
	fout << setprecision(5) << fabs(area)/2.0;
}

// Run program: Ctrl + F5 or Debug > Start Without Debugging menu
// Debug program: F5 or Debug > Start Debugging menu

// Tips for Getting Started: 
//   1. Use the Solution Explorer window to add/manage files
//   2. Use the Team Explorer window to connect to source control
//   3. Use the Output window to see build output and other messages
//   4. Use the Error List window to view errors
//   5. Go to Project > Add New Item to create new code files, or Project > Add Existing Item to add existing code files to the project
//   6. In the future, to open this project again, go to File > Open > Project and select the .sln file