using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;

class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[][] NArr(long n) => Enumerable.Repeat(0, (int)n).Select(_ => NList).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var map = NArr(n);
        var ans = 0L;
        for (var i = 0; i < n; ++i)
        {
            var j = (i + 1) % n;
            ans += (map[i][1] + map[j][1]) * (map[i][0] - map[j][0]);
        }
        WriteLine(ans);
    }
}