/*#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

struct vertex{
	int x, y;
}a[100010];

double s;

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

#include <bits/stdc++.h>
using namespace std;

int n;
double ans;

struct point{
    int x, y;
} p[100010];

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++){
        ans += (p[i].x * p[i+1].y - p[i+1].x * p[i].y);
    }
    cout << fixed << setprecision(5) << ans/2;
    return 0;
}