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

using namespace std;

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

double x[100010], y[100010], s;

int main(){
	int n, i;
	
	in >> n;
	
	for(i = 1; i <= n; i++)
		in >> x[i] >> y[i];
		
	x[n + 1] = x[1];
	y[n + 1] = y[1];
	
	for(i = 1; i <= n; i++)
		s += (0.5 * (x[i] * y[i + 1] - x[i + 1] * y[i]));
		
	if(s < 0)
		s = -s;
		
	out << fixed << setprecision(5) << s;
	
	return 0;
}