#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("aria.in");
ofstream out("aria.out");

struct vertex{
	int x, y;
}a[100010];

double s;

int main(){
	int n, i;
	
	in >> n;
	
	for(i = 1; i <= n; i++)
		in >> a[i].x >> a[i].y;
		
	a[n + 1] = a[1];
	
	for(i = 1; i <= n; i++)
		s += (a[i].x * a[i + 1].y - a[i].y * a[i + 1].x);
		
	out << fixed << setprecision(5) << s / 2;
	
	return 0;
}