#include <bits/stdc++.h>
#define X first
#define Y second

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
pair<long double, long double> coord[100002];
int n;

int main()
{
	fin >> n;
	
	for(int i = 1; i <= n; i++)
		fin >> coord[i].X >> coord[i].Y;
	
	coord[n + 1] = coord[1];
	long double sol = 0;
	
	for(int i = 1; i <= n; i++)
		sol += coord[i].X * coord[i + 1].Y - coord[i].Y * coord[i + 1].X;
		
	sol = abs(sol) * 1 / 2;
	fout << setprecision(5) << sol;
	return 0;
}