#include <iostream>
#include <fstream>
#include <vector>
#include <cmath>
#include <cstdio>

using namespace std;

int n;
vector<pair<double,double> > points;

int main()
{
	freopen("aria.in", "r" , stdin);
	freopen ("aria.out", "w", stdout);
	scanf ("%d",&n);
	double area = 0;
	for (int i = 1; i <= n; ++i)
	{
		double x,y;
		//fin>>x>>y;
		scanf("%lf %lf", &x, &y);
		points.push_back(make_pair(x,y));
	}
	for (int i = 0; i <= n-2; i++)
	{
		area += (points[i].first * points[i+1].second - points[i+1].first * points[i].second);
	}
	area += (points[n-1].first * points[0].second - points[0].first * points[n-1].second);
	area = area/2.0;
	printf("%lf", area);
}