#include <bits/stdc++.h>

using namespace std;

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


int main()
{
pair<double,double>v[100005];
	int n;
	double s=0;
	in>>n;
	for(int i=0;i<n;i++)
		in>>v[i].first>>v[i].second;
	v[n].first=v[0].first;
	v[n].second=v[0].second;
	for(int i=0;i<n;i++)
		s+=(v[i].first*v[i+1].second-v[i+1].first*v[i].second);
	out<<fixed<<setprecision(5)<<s/2.0;
	return 0;
}