#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
 
int main() { 
	int n, x1, x2, y1, y2, x, y; 
	double sum=0; 
	cin >> n >> x >> y; 
	x1=x; 
	y1=y; 
	for (int i=0; i<(n-1); i++) { 
		cin >> x2 >> y2; 
		sum=sum+(x1+x2)*(y2-y1); 
		x1=x2; 
		y1=y2; 
	} 
	sum=sum+(x+x2)*(y-y2); 
	cout << fixed << setprecision(3) << abs(sum) / 2 << endl;
	return 0; 
}