#include <bits/stdc++.h>
#define ld long double

using namespace std;

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

struct my{
	ld x, y;
};

int n; my a[100100]; ld ans = 0;

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