#include <vector>
#include <fstream>
#include <iostream>


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

int N;
std::vector<std::pair<int, int> > points;

int area(std::pair<int, int> p1, std::pair<int, int> p2) {
	return p1.first * p2.second - p2.first * p1.second;
}



int main()
{
	fin >> N;
	for (int i = 0; i < N; i++) {
		int x, y;
		fin >> x >> y;
		points.push_back(std::make_pair(x, y));
	}
	
	int sum = 0;
	
	for (int i = 0; i < N - 1; i++) {
		sum += area(points[i], points[i + 1]);
	}


	fout << sum << '\n';
    return 0;
}