#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
typedef long long ll;

int n;
pair<double,double> A[100010];
double ans;

int main(){
	ifstream cin ("aria.in");
	ofstream cout ("aria.out");
	cin>>n;
	for (int i=1; i<=n; i++){
		cin>>A[i].x>>A[i].y;
	}
	A[0]=A[n];
	for (int i=0; i<n; i++){
		ans+=(A[i].x*A[i+1].y-A[i+1].x*A[i].y);
	}
	cout<<setprecision(5)<<abs(ans/2.0);
	return 0;
}