#include <bits/stdc++.h>

using namespace std;

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

int n;
double x[100100], y[100100], ans;

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