#include <bits/stdc++.h>
using namespace std;

struct point{
	double x,y;
};

double det(point a, point b, point c){
	return(a.x * b.y) + (b.x * c.y) + (c.x * a.y) - (b.y * c.x) - (c.y * a.x) - (a.y * b.x);
}

int main(){
	ifstream cin("aria.in");
	ofstream cout("aria.out");
	int n;
	cin >> n;
	point arr[n + 2];
	for(int i=0;i<n;i++)
		cin >> arr[i].x >> arr[i].y;
	double ans = 0;
	for(int i=2;i<n;i++)
		ans += det(arr[0], arr[i - 1], arr[i]) / 2;
	cout << fixed <<  setprecision(6) <<  ans;
	return(0);
}