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

int n;
pair<int,int> 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;
	}
	for (int i=1; i<=n; i++){
		ans+=0.5*(A[i].x*A[(i==n?1:i+1)].y-A[(i==n?1:i+1)].x*A[i].y);
	}
	cout<<abs(ans);
	return 0;
}