using static System.Math;
using System;

public class Hello
{
    static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        n--;
        var x = new int[n];
        var y = new int[n];
        string[] line = Console.ReadLine().Trim().Split(' ');
        var x0 = int.Parse(line[0]);
        var y0 = int.Parse(line[1]);
        for (int i = 0; i < n; i++)
        {
            line = Console.ReadLine().Trim().Split(' ');
            x[i] = int.Parse(line[0]) - x0;
            y[i] = int.Parse(line[1]) - y0;
        }
        getAns(n, x, y);
    }
    static long calc(int[] x, int[] y, int i)
    {
        long ans = Abs(x[i] * y[i + 1] - x[i + 1] * y[i]);
        return ans;
    }
    static void getAns(int n, int[] x, int[] y)
    {
        long ans = 0;
        for (int i = 0; i < n - 1; i++) ans += calc(x, y, i);
        Console.WriteLine(ans); ;
    }
}