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

int n;

double rs,x1,y11;
pii a,b;

double cross_product(pii a, pii b) {
	return a.x*b.y-a.y*b.x;
}


int main() {
	ifstream cin("aria.in");
	ofstream cout("aria.out");
	cin>>n;
	
	if (n<=2) {
		cout<<0; return 0;
	}
	 
	cin>>x1>>y11; b={x1,y11};
	for (int i=1; i<n; i++) {
		cin>>a.x>>a.y;
		rs+=(cross_product(b,a)); 
		b=a;
	} 
	cout<<fixed<<setprecision(5)<<0.5*abs(rs);
	
	return 0;
}