Problem:
Score : 300 points

Problem Statement

N persons are standing in a row. The height of the i-th person from the front is A_i.

We want to have each person stand on a stool of some heights - at least zero - so that the following condition is satisfied for every person:

Condition: Nobody in front of the person is taller than the person. Here, the height of a person includes the stool.

Find the minimum total height of the stools needed to meet this goal.

Constraints

1 \leq N \leq 2\times 10^5

1 \leq A_i \leq 10^9

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N
A_1 \ldots A_N

Output

Print the minimum total height of the stools needed to meet the goal.

Sample Input 1

5
2 1 5 4 3

Sample Output 1

4

If the persons stand on stools of heights 0, 1, 0, 1, and 2, respectively, their heights will be 2, 2, 5, 5, and 5, satisfying the condition.

We cannot meet the goal with a smaller total height of the stools.

Sample Input 2

5
3 3 3 3 3

Sample Output 2

0

Giving a stool of height 0 to everyone will work.

Solution in C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Linq;
using System.Numerics;
using System.Collections.Generic;
using System.IO;
using System.Text;
using N = System.Int64;

static class Program
{
    static public void Main(string[] args)
    {
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        var solver = new Solver();
        solver.Solve();
        Console.Out.Flush();
    }
}

public class Solver
{
    public void Solve()
    {
        N n = rn;
        N a = 0;
        N m = 0;//前側の中で最高
        N p = 0;
        for(int i = 0; i < n; i++) {
            a = rn;
            if (a >= m)//高さ更新
            {
                m = a;
            }
            else
            {
                p += (m - a);
            }
        }
        Write(p);
    }

    N rn { get { return sc.Integer(); } }
    double rd { get { return sc.Double(); } }
    string rs { get { return sc.Scan(); } }
    public StreamScanner sc = new StreamScanner(Console.OpenStandardInput());

    /*
    private const N mod = 1000000007;
    N Reverse(N n) { return Pow(n, mod - 2); }
    N Mul(params N[] n) { N p = n[0]; for (N i = 1; i < n.Length; i++) { p = (p * n[i]) % mod; } return p; }
    N Pow(N n, N p) { N t = n, q = 1; while (p > 0) { if (p % 2 == 1) { q = (q * t) % mod; } t = (t * t) % mod; p >>= 1; } return q; }
    N PowUpper(N n, N b) { N p = 1; while (p <= n) { p *= b; } return p; }
    N Sign(N n) { return n == 0 ? 0 : n > 0 ? 1 : -1; }
    N Abs(N n) { return (n > 0) ? n : -n; }
    N Gcd(params N[] n) { N p = n[0]; for (N i = 1; i < n.Length; i++) { p = Gcd(p, n[i]); } return p; }
    N Gcd(N a, N b) { return (a == 0) ? b : Gcd(b % a, a); }
    N Lcm(params N[] n) { N p = n[0]; for (N i = 1; i < n.Length; i++) { p = Lcm(p, n[i]); } return p; }
    N Lcm(N a, N b) { return (a / Gcd(a, b) * b) % mod; }*/

    void Write<T>(T t) { Console.WriteLine(t.ToString()); }
    void YN(bool t) { Console.WriteLine(t ? "YES" : "NO"); }
    void Yn(bool t) { Console.WriteLine(t ? "Yes" : "No"); }
    void yn(bool t) { Console.WriteLine(t ? "yes" : "no"); }
}



public class StreamScanner
{
    public StreamScanner(Stream stream) { str = stream; }
    private readonly Stream str;
    private readonly byte[] buf = new byte[1024];
    private int len, ptr;
    public bool isEof = false;
    public bool IsEndOfStream { get { return isEof; } }
    private byte read()
    {
        if (isEof) throw new EndOfStreamException();
        if (ptr >= len)
        {
            ptr = 0;
            if ((len = str.Read(buf, 0, 1024)) <= 0)
            {
                isEof = true;
                return 0;
            }
        }
        return buf[ptr++];
    }
    public char Char()
    {
        byte b = 0;
        do b = read();
        while (b < 33 || 126 < b);
        return (char)b;
    }
    public string Scan()
    {
        var sb = new StringBuilder();
        for (var b = Char(); b >= 33 && b <= 126; b = (char)read())
            sb.Append(b);
        return sb.ToString();
    }
    public N Integer()
    {
        N ret = 0; byte b = 0; var ng = false;
        do b = read();
        while (b != '-' && (b < '0' || '9' < b));
        if (b == '-') { ng = true; b = read(); }
        for (; true; b = read())
        {
            if (b < '0' || '9' < b)
                return ng ? -ret : ret;
            else ret = ret * 10 + b - '0';
        }
    }
    public double Double() { return double.Parse(Scan()); }
}