#include <iostream>
#include <cstdio>
#include <vector>
#include <cmath>
using namespace std;

struct Point
{
	double x, y;
};

int main()
{
	int n;
	cin >> n;
	vector<Point> a(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i].x >> a[i].y;
	}
	double area = 0;
	for (int i = 0; i < n; i++) {
		int j = (i+1)%n;
		area += a[i].x*a[j].y-a[i].y*a[j].x;
	}
	area = abs(area)/2;
	printf("%f\n", area);
	return 0;
}