#include <bits/stdc++.h>
#define x first
#define y second
#define PII pair<double, double>

using namespace std;

int n, cnt;
PII p[125000];
long double rs;

int main(){
	ifstream cin("aria.in");
	ofstream cout("aria.out");
	cin >> n;
	for (int i = 1; i <= n; i++) cin >> p[i].x >> p[i].y;
	p[n+1] = p[1];
	for (int i = 1; i <= n; i++)
	    rs += p[i].x * p[i+1].y - p[i+1].x * p[i].y;
	cout << setprecision(6) << fixed << rs / 2.0;
    cerr << "Fucking time elapsed: " << clock() * 1000.0 / CLOCKS_PER_SEC << " ms" << '\n';
    return 0;
}