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 char[][] SList(long n) => Enumerable.Repeat(0, (int)n).Select(_ => ReadLine().ToCharArray()).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var a = new int[n];
        for (var i = 0; i + 1 < n; ++i) a[i] = NN;
        var cur = 0;
        var ans = 0;
        for (var i = 0; i < n; ++i)
        {
            var c = NList;
            cur += c[1] - c[0];
            ans += a[i] * cur;
        }
        WriteLine(ans);
    }
}