#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
pair<double,double> f,a,b;
double sol;
int N;
int main(){
	ifstream cin("aria.in");
	ofstream cout("aria.out");
	cin >> N >> f.x >> f.y;
	a = f;
	while(N-- > 1){cin >> b.x >> b.y; sol+=a.x*b.y - a.y*b.x;swap(a,b);}
	sol += a.x*f.y - a.y*f.x;
	cout << sol/2;
	return 0;
}