#include <bits/stdc++.h>

using namespace std;

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

int n;
double s;
pair <int, int> v[100005];

int main(){
	fin >> n;
	for(int i=0;i<n;i++)
		fin >> v[i].first >> v[i].second;
	for(int i=0;i<n-1;i++)
		s += 0.50000 * (v[i].first * v[i+1].second - v[i].second * v[i+1].first);
	fout << fixed << setprecision(5) << s;
	return 0;
}