#include <bits/stdc++.h>

using namespace std;

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

const int DIM = 1e5 + 2;

using ld = long double;

pair <ld, ld> v[DIM];

main()
{
	int n;
	fin >> n;
	
	for(int i = 1; i <= n; ++i)
	{
		fin >> v[i].first >> v[i].second;
	}
	
	v[++n] = v[1];
	
	ld ans = 0;
	
	for(int i = 2; i <= n + 1; ++i)
	{
		ans += v[i - 1].first * v[i].second - v[i - 1].second * v[i].first;
	}
	
	if(ans < 0)
		ans = -ans;
	
	ans /= 2;
	
	fout << fixed << setprecision(7) << ans << '\n';
}