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

struct Point {
	double x, y;
};

double Product(Point a, Point b) {
	return a.x * b.y - b.x * a.y;
}

int main () {
	ifstream f("aria.in");
	ofstream g("aria.out");
	
	int n; f >> n;
	vector<Point> v(n);
	for(int i=0; i<n; i++) f >> v[i].x >> v[i].y;
	v.emplace_back(v[0]);
	double area = 0;
	for(int i=0; i<n; i++) 
		area += 1LL * Product(v[i], v[i+1]);
	g << double(area / 2.00);
	return 0;
}